Hi @madmax12 ,
most probably you have a search module wich has this override where the function is called but not declared anymore.
One way to solve this would be to uninstall the search module, then clear cache, and if the error is still there then most probably you have an override wherethis function is called, maybe something like a file override/controllers/front/listing/SearchController.php
From the 500 error page there is mentioned the call stack f the php files/functions when you hover over the file names on the error page.
Thy to find the override file which is calling the "doProductSearchPos" and rename the override to not have the .php extension, or delete it(after backing it up)
If you've found the file then that override should be checked and only the problematic function to be deleted by a developer.
Let me know if you manage to find it.
Kind regards, Leo.