Luckily I don't test in a production environment, but the exact same thing happens to me. If Datakick needs a test environment, where to check the bug, I can provide access without problem.
I think the bug is due to the change in the way the new versions store passwords.
The failure when upgrading is as follows:
And the log after trying to log into the admin panel with the correct username and password:
PHP Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /home/test/public_html/classes/Tab.php on line 664
PHP Warning: Undefined array key "signature" in /home/test/public_html/classes/Employee.php on line 557