We are running vB 5.6.4. I intended to upgrade to vB 5.6.4 PL1, but get this error.

A System Error has occured. The software is experiencing a systems error. You should attempt to repeat your last action. If this error occurs again, please contact the site administrator.

I restored the live site from backup, downloaded it to my xampp testing environment a starting to examine. The error appears also on the testing environment. It happens when I clear system cache in admincp’s maintenance, so before I installed the PL1.
When I enable the debug mode, I get this info:
Error :Call to a member function fetch_userinfo() on null on line 1781 in C:\Xampp\xampp7.3.19_3_c\htdocs\divne\core\includes\functions.php
Step 0: showErrorPage() called on line 382 in C:\Xampp\xampp7.3.19_3_c\htdocs\divne\includes\vb5\applicationabstract.php
Step 1: handleException() called
Exception No Types Found on line 219 in phar://C:/Xampp/xampp7.3.19_3_c/htdocs/divne/core/vb/vb.phar/types.php

Any idea what to do?

My real forum configuration is:
Server type: linux
Web server: Apache v2.4.38
PHP: 7.3.19-1~deb10u1
MySQL Version: 10.3.25-MariaDB-0+deb10u1

My local testing environment is:
Server type: WINNT
Web server: Apache v2.4.43
PHP: 7.3.19
MySQL Version: 10.4.13-MariaDB