@codeispoetry: Thanks! I indeed applied severals security measurements via Plesk and unfortunately a few cannot be reverted, so I am somewhat lost where to find the cause of the error. — I installed Zotero with the same security measurements with an other WP-installation on the same server and there everything works smoothly. I checked the permissions and they should be ok. I also changed the default database table prefix, which could be the cause. I fear I need to completely re-install my site, if I do not find the error in due time…
