P.s. I recorded the following when trying to logout, maybe it helps:

I now found out, that it works when SEO-Friendly URLs is deactivated...!!? Hope that helps.

Hello, thank you for sharing that the issue has been solved. Sometimes, some incomplete settings of multilingual on your site might cause an issue. For example, you can also check if you have renamed renamed htaccess.txt to .htaccess from your joomla root directory. Possible problems can also occur when your SEF/SEO settings is not fully configured. You can also follow this blog, for future. The steps here are helpful. https://www.joomshaper.com/blog/step-by-step-tutorial-for-creating-a-multilingual-joomla-site https://www.joomshaper.com/blog/make-your-joomla-site-seo-friendly

Hi, Sorry for the confusion, it is not resolved, since I want to keep the SEO friendly setting ON. I have two files in my ROOT: htaccess.txt AND .htaccess, isn't that strange??? Both same size... I will also read the blogs you mentionned, thanks.

Sure, after you go through the blogs and recheck if every steps have been followed properly, you can let us know if the issue still exists. We will help you :)