Missing boundary in multipart/form-data POST data in Unknown on line 0

[q2apro]

Describe the bug
Found the PHP warnings in the server log.

PHP Warning:  Missing boundary in multipart/form-data POST data in Unknown on line 0
PHP Warning:  session_start(): Cannot start session when headers already sent in /srv/users/matheretter/apps/mathelounge/public/qa-include/app/users.php on line 162
PHP Warning:  Cannot modify header information - headers already sent in /srv/users/matheretter/apps/mathelounge/public/qa-include/qa-page-not-found.php on line 30
PHP Warning:  Cannot modify header information - headers already sent in /srv/users/matheretter/apps/mathelounge/public/qa-include/app/users.php on line 1335
PHP Warning:  Cannot modify header information - headers already sent in /srv/users/matheretter/apps/mathelounge/public/qa-include/app/page.php on line 357

What could be the culprit? How to avoid the error/warning in the first place?