Is the site live to have a look?

The URLs seem fine, e.g. "/contactadvertenties/mannen/energieke-man-55-uit-zuid-holland". Maybe it was some caching issue?

Scratch that, thought it was all URLs, but you only meant the Categories list from K2 Tools. I'll have a look and let you know.

I had a look again at the advanced SEF router code... To be honest it's kind of a mess what was previous there and extremely bad performance-wise. I had adapted the code in K2 v2.10.3 but this is a side-effect unfortunately which I'll note as a bug and properly check on the coming updates of K2.

In the meantime, I would propose this:
- Set "Enable custom SEF settings for K2 URLs" to "No".
- Ensure your categories are properly mapped into a menu. If there are many and you don't want to do that, at least map the "parent" ones and *ensure* there is at least a single K2 menu item pointing to all categories with a menu alias that is generic (e.g. "show"). This menu item does not need to be shown in the frontend of your site (it can be in some hidden menu).
- Finally clear your caches and check your URLs again.

For the record, upcoming changes in K2's code will be massive so as to avoid any release hiccups in the future (whenever the J team feels like making breaking changes). This includes the URL router which will be re-written from scratch.

I've actually gone ahead and updated the URL router. Version 2.11.20230323 is now available to upgrade and should resolve any such issues.

I've also enabled advanced SEF URLs on the K2 demo site here: demo.getk2.org/