Okay, so I've tried my installation ro another server with no luck. I've updated to K2.5.4, sadly, still no luck.

On the other hand, when I installed K2.5.4, and searched for a k2-item the 'error' changed to 'line 935'. The code line 935 is however the same code as before, just moved up a bit.

If there is no solution for this problem, can another module or component maybe do the same thing as K2 Tool Search?

Thanks in advance.

Hello, Linus.

Try to check it without joomfish install.

For alternative tool try to use this extention:
getk2.org/extend/extensions/date/item/183-k2filter

Thanks for the suggestion Andrey, the problem is just that I don't have JoomFish installed and never had.
The extention however, is it just a "filter" tool or is there a search function aswell?

The extension provide an alternative search functionality.