I found a possible reason why the (well known) admin loop might occur. An other problem is that changes to the general settings are NOT saved into the db either

The anoyance is that --when logged in as adminuser-- a click on the "administration menu" causes a loop to enter that only shows "This is your first time to enter the administration section."

Several solutions posted around here go from disabling the firewall (absurd) to modding the code (even more absurd), or changing browser (totaly nuts)

My problem started when adding a custom theme to the directory ./xoops_path/themes/

SOLLUTION:
If you have the loop problem, then first check next things:
- the new theme directory AND its files inside MUST have public read access (typicaly chmod 755)

- make sure ALL needed files are in that theme !!!!
if even 1 gif file is missing, the loop problem will start.
read other documentation on how to make your own themes if needed.

advisable is to move any added themes to a different location on disk and test one at a time, so you know which theme is causing the trouble

As said at start, this might be only 1 of the possible scenarios that causes the loop to start.
good luck ...