Gallery on public link: next and back buttons not working after upgrade to NC12

I just upgraded from NC11 to NC12 and now the next and back buttons don’t work in the gallery, but the problem only occurs on public shared links, like this one:
When I’m logged on it does work.
I also noticed (but could not really replicate it) there was a moment where I tried to stop sharing a folder, it immediately got shared again (so could not remove the shared link) but after a while and possibly another server reboot that seems to work again.
After the NC12 upgrade I also re-enabled opcache in php.ini, as proposed in the admin settings. Disabling the opcache (and rebooting) does not change the described behaviour.
PHP version is 7.0.