Users with old session cookie versions are not handled correctly
https://sentry.fsmpi.rwth-aachen.de/protokollsystem/protokollsystem/issues/11/
ValueError: could not convert string to float: 'False'
(8 additional frame(s) were not displayed)
...
File "program/server.py", line 223, in index
user = current_user()
ValueError: could not convert string to float: 'False'