User appears logged out when clicking thumbnail

I run the latest version of NGG and WP. I am facing the problem that when I click a thumbnail in a page-embedded gallery, the user appears as if he was logged out. However, if the user goes back in browser history or resumes browsing some other pages on the blog, he is still logged in.

I have no idea why it does not work anymore, nothing changed. I also checked the error log, nothing is related to this issue. Memory limit is 256M, also apc is set to 256M.

Tried to reinstall, no success. Any hints on what to do?

https://www.ads-software.com/plugins/user-access-manager-nextgen-gallery-extension/