Leonid, thanks for your reply. I had - as usual - some problems with modSecurity and deactivated those rules on my server. Now it doesn't show any more active security rules, except of one, which I consider a master rule for vServers at my hosting server, because it cannot be deactivated. The server's error log still reports a security rule violation, which might be very likely the problem. Please see my current error.log in the attachment.
Thanks for checking my server, LeonidS. So far we know, that I have to talk to my server host, since I run UNA on a shared virtual server environment, that might prevent modules from functioning as expected.