can't select first item of Select2 enabled list

[atopcu]

I see first item on field when field is empty with gray color:

But you can't select it when you want.

Environment:
Redmine version 2.6.6.stable.14445
Ruby version 2.0.0-p481 (2014-05-08) [x86_64-linux]
Rails version 3.2.22
Environment production
Database adapter Mysql2