404 error on CERTAIN categories with SEF urls enabled

So I was browsing through my website and opening/filtering a few products for further edits on the backend when I encountered this extremely weird error.

CERTAIN categories/subcategories throw a 404 error, only on one specific language ( I use greek and english, greek being the main one ) and with ONLY a certain keyword as their url ( basically, their auto-generated keyword, or the "default" keyword that is given after the category name ).

So let's say I have category1 category2 category3 as category names. The greek url is an english version of the greek name so it's different than the english one a lot.

These certain categories throw a 404 error when accessed, no matter what. I deleted the url alias table, set the url by hand ( I am using the Complete Seo Package extension ) inside the category settings, tried deleting the seo keyword from inside the database, removed duplicate urls ( using the extension but also checking by hand ) and nothing changed.

The same categories, work fine when switching the website to the English version. They also work fine when I change their keyword url completely, for example when putting TEST as a url keyword, it works. If I put category1111111 as a keyword, it still doesn't work.

I really can't understand what's happening. All caches are disabled and deleted ( journal + lightning ), the extension was reinstalled from scratch after uninstalling it, but still nothing.

My issue is similar to this one : viewtopic.php?f=20&t=28983

but I don't have duplicate entries inside the url alias table whatsoever. I even tried creating a new category but with the same name after having renamed the old one, in case there's a conflict with the category id somewhere, but still the same issue.

Apache, htaccess and all settings are enabled and configured properly since day 1, and haven't changed them since. I just discovered the problem out of the blue.

Any suggestions?

It might help if you post a link to the site?

Hello!

I replied via PM.

I have disabled SEF urls for the time being because it's a live site and I can't risk my customers receiving 404 errors when trying to browse.

But there are a lot of internal links and google searches pointing to the SEF urls that are throwing 404 errors and I can't afford to have the issue unresolved for long.

Another check I did, is if I put the category URL in greek characters, it works. But I don't want it in greek since everything so far is with english characters.

So the combination of certain categories with their sef urls that are generated by their name, throw the 404 error.