Maybe this is related to other problem found in JoomSEF 4.2.9 - the language switcher module caused a different language than the current one to be loaded and it wasn't switched back afterwards.
This problem is fixed in JoomSEF 4.3.0, so try updating your JoomSEF, maybe it will fix your problem too.

Of course having a backup of your website before updating any component is recommended.

I have been installing the update en purging all urls. Been checking it for the past few weeks but it keeps going wrong.
Same issue still.

The weird thing is that I can't get my finger on where it goes wrong. Each time I purge the urls after the links go wrong again, new ones will be created, it goes correctly for a day or two and then it starts to create weird links.

For the Dutch menu for example when navigating to a submenu, the homepage might link correctly to the /nl page.
For other subpages, the homepage link directs /en
After I purge all sef urls and it goes wrong again after two days, the homepage of other supages link back to /nl or /en
It's like different every time.

What consequently goes wrong is the German menu. When at a subpage, the homepage always seems to direct back to /nl instead of /de

Hi,

Do you use cache in Joomla? If you do, try disabling it and check if it helps.

Other than that I don't know what could be causing this problem, we haven't encountered this yet. I can offer you our paid support service to check it directly on your website: www.artio.net/e-shop/support-services/joomla-related-support-1h