We run a custom WP theme with some plugins including Yoast SEO premium and WPML.
We also run a version without WPML.

In the version WITH WPML subcategories (quite randomly) give back a 404. This seems to happen when anyone is working in the backend (not sure if actual post edits have to be made or just login is enough).

At random subcategories seem te go down (post, pages and categories work). Not all the subacategories go down however.

When I resave the permalink structure (without any changes) the 404 subcategories seem to work again but others go down.

When I save permalinks twice they all seem to work until…?
I’m not 100% sure that saving the permalinks twice in a short time is the real "fix".

We are running into this problem for over a year now without finding a fix.

Anyone with a clue whats going on?

(the site without WPML never has this problem)