Issue: can't login on latest RC on Debian 9.4 with PHP7.1
Cause: session not working
After searching a lot and finding nothing to help with this error I discovered that starting with PHP7 the null values are not accepted anymore, a string has to be returned on a read function.
The fix is to change the return value of the db_read function in rcube_session.php (~ line 185) from null to ''.
Hope it helps!