[5.x]: Server error when trying to login

[JulieVelghe]

What happened?

Description

When I or the client tries to login to the backend of our production and staging website, we get a 'there was a server error'. Locally everything seems fine.
We didn't do anything special last week. We did activate varnish caching, but that was longer than a week ago, and I believe that the admin url was specifically left out of the varnish caching. If there was a problem with that, I would presume we would have gotten this error sooner.

Steps to reproduce

1. surf to the /admin url
2. try to login

Expected behavior

we can login to the backend

Actual behavior

we get a 'server error' and remain on the login screen.

Craft CMS version

5.9.22

PHP version

8.4

Operating system and version

No response

Database type and version

MySQL 8.0.40

Image driver and version

Imagick 3.8.1 (ImageMagick 7.1.1-43)

Installed plugins and versions

"carlcs/craft-diywidget": "4.1.1",
"craftcms/ckeditor": "4.11.3",
"craftcms/cms": "5.9.22",
"craftcms/element-api": "4.2.0",
"craftcms/feed-me": "6.11.0",
"craftcms/redactor": "4.2.0",
"doublesecretagency/craft-cpcss": "3.0.0",
"elasticsearch/elasticsearch": "v7.17.2",
"ether/seo": "5.0.0",
"nystudio107/craft-code-editor": "1.0.27",
"putyourlightson/craft-entry-count": "4.0.0",
"putyourlightson/craft-sendgrid": "3.0.0",
"putyourlightson/craft-sprig": "3.7.2",
"solspace/craft-freeform": "5.15.8",
"spacecatninja/imager-x": "5.2.1",
"spicyweb/craft-embedded-assets": "5.4.7",
"utakka/redactor-anchors": "1.5.0",
"venveo/craft-compress": "5.0.0",
"verbb/default-dashboard": "3.0.2",
"verbb/super-table": "4.0.5",
"verbb/timber": "2.0.4",
"vlucas/phpdotenv": "v3.6.10"

[linear-code]

CMS-2184

[JulieVelghe]

This is a piece of the web error log: 2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":7617816}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":15636856}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":15637200}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16974288}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16974792}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16991344}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16991720}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16992064}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16992408}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16992752}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16993760}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16994104}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16994448}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16994792}
2026-06-08 13:37:26 [web.ERROR] [yii\web\Session::open] session_start(): Failed to read session data: files (path: /data/sites/web/i-mens2026be/tmp) {"memory":16995456}
2026-06-08 13:37:26 [web.ERROR] [yii\base\Exception] No user session token exists. {"trace":["#0 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/yiisoft/yii2/web/User.php(698): craft\elements\User->getAuthKey()","#1 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/web/User.php(479): yii\web\User->switchIdentity(Object(craft\elements\User), 3600)","#2 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/yiisoft/yii2/web/User.php(270): craft\web\User->switchIdentity(Object(craft\elements\User), 3600)","#3 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/web/User.php(403): yii\web\User->login(Object(craft\elements\User), 3600)","#4 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/controllers/UsersController.php(364): craft\web\User->login(Object(craft\elements\User), 3600)","#5 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/controllers/UsersController.php(303): craft\controllers\UsersController->_completeLogin(Object(craft\elements\User), 3600)","#6 [internal function]: craft\controllers\UsersController->actionLogin()","#7 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/yiisoft/yii2/base/InlineAction.php(66): call_user_func_array(Array, Array)","#8 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/yiisoft/yii2/base/Controller.php(197): yii\base\InlineAction->runWithParams(Array)","#9 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/yiisoft/yii2/base/Module.php(554): yii\base\Controller->runAction('...', Array)","#10 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/web/Application.php(368): yii\base\Module->runAction('...', Array)","#11 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/web/Application.php(635): craft\web\Application->runAction('...', Array)","#12 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/web/Application.php(330): craft\web\Application->_processActionRequest(Object(craft\web\Request))","#13 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/yiisoft/yii2/base/Application.php(391): craft\web\Application->handleRequest(Object(craft\web\Request))","#14 /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/www/index.php(21): yii\base\Application->run()","#15 {main}"],"memory":16737176,"exception":"[object] (yii\base\Exception(code: 0): No user session token exists. at /data/sites/web/i-mens2026be/checkout/staging/3cbb2dbf3ce00215fece38f96e93de111ea00714/vendor/craftcms/cms/src/elements/User.php:1299)"}

[JulieVelghe]

The only thing we did was going from php 8.2 to php 8.4, but that shouldn't break the admin?

[JulieVelghe]

Some extra info: we deactivated the varnish caching, but the problem persisted. In the devtools of safari, I see an index.php in the urls, and a status 500, but that is probably because of those session errors

[JulieVelghe]

A colleague found out that the folder where the sessions files where being kept (the tmp folder), was overflowing. There were around 170k of files in there, with the bulk of them being sess_ files. Removing them temporarily gives us access back to the back-end, but we have to find out why files are being written there and how it is possible for that folder to get full.
Do you have any suggestions? Are there any updates that can mitigate this issue? Or is there some sort of configuration we can do?