After using

tail -f /var/logs/apache2/error.log

i could see that the selfoss.db was not writeable for the script that controls read/unread.

I fixed it with

sudo chown www-data:www-data /var/www/selfoss/data/sqlite/selfoss.db

After that i could directly and wihtout any problem mark items as un/read.

hope that helps someone in the same situation.

br