Slim v2 & v3 Fatal Error Slim\Helper\Set.php

Hi, since couple of days I have a strange error which show up from time to time and disappears later.
Usually after couple of minutes everything is fine again.
The message is:
Fatal error in C:\xampp\htdocs\hiabreports\vendor\slim\slim\Slim\Helper\Set.php

Only thing which I changed is that increase performance on xampp server (memory limit 128M, max post size 8M, upload max size 4M)

Honestly I don`t know what to do… what could cause that. My applications works fine for couple of years now…

Please provide the whole stack trace report.

[Mon Feb 17 14:14:48.973040 2020] [:error] [pid 2732:tid 1824] [client] PHP Fatal error: Class Slim\Helper\Set contains 5 abstract methods and must therefore be declared abstract or implement the remaining methods (ArrayAccess::\xf0:e\x01\x03, ArrayAccess::\xa8\x80h\x01\x03, ArrayAccess::H\x83h\x01\x03, …) in C:\xampp\htdocs\hiquality\vendor\slim\slim\Slim\Helper\Set.php on line 35