Problems with login of many students

Good morning. 

Lately, we have started to have problems with many studentes. We use chromebooks in the school.

When they try to log in, they get a message "There was a problem with your form submission.

Please wait 300 seconds and try again. "

After waiting more than 300 seconds, the problem doesn't get solved. Even after some days :(

Is there anything I can do to fix this?

Thank you in advance.