Failed user login on production server using Symfony framework (Authentication request could not be processed due to...)
It looks like that the error:
Authentication request could not be processed due to a system problem.
is too generic and does not tell anything about where the problem is (there is an issue opened about this matter here).
I solved my issue by checking the logs and see what happened (in var/logs/dev.log), hoping this helps someone.
In my specific case, there was a wrong parameter in parameters.yml about database connection. But, again, the error is too generic and does not necessarily imply that the problem is related with database connection.