Hello,
Thanks for your reply, I have been able to resolve it.
It turned out that Joomdle (bridge extension for Joomla-Moodle) was the cause. I had found this out almost two years ago as well, and had posted about it on the Joomdle support forums. I just totally forgot I had done that, shortly afterwards my personal life became very hectic. But just in case anyone else might experience the same issue (or if this happens to me again in a few years and by then I forgot again lol), I'll post the solution here.
If Joomla and Moodle are on the same domain (which is the case for me) and cURL is enabled, you need to enable "redirectless logout" in Joomdle's configuration in Joomla.
The issue resurfaced now since I have Mobile Joomla configured to have mobile users go to the m. of my domain instead of www.
For that reason, I had disabled the "redirectless logout" again (not remembering why I had enabled it in the first place). Re-enabling apparently doesn't interfere with Joomdle, even when Joomla is being accessed on the m. subdomain (and Moodle still on the www.).
The redirect on logout, however, created an incorrect URL and therefore a 404 page (only when on the mobile subdomain though). This however can be fixed by making sure that the logout URL in the CB Login module (the one used for the mobile site) is the non-SEF URL starting with index.php.
In fact (and this is just a sidenote), providing a SEF-url for logout redirection in the CB Login works in itself just fine. Only in my case, on the mobile version of Joomla I have to provide a non-SEF url due to Joomla.
Thanks for your help - and hopefully this information might also be helpful to others.