And now it hits me: I haven't had any log entries in mod_security until yesterday. That's why I didn't have any problems before with the same config. I suspect a new site is triggering mod_security.
My server is also quite new (23 feb). What license are you running on? Trial or full license? I changed to a full license two weeks ago, because my trial would have expired yesterday (the same day the problems started...)
First I thought it was related, because I didn't change anything Apache...
That's 'funny', I have the exact same problem since yesterday. This morning, it happend for the second time. Same config (Debian 8.3, Plesk 12.5.30 Update #24). I've made the modification and will report back.
Did you just install this server, or was it already running a while?
Regarding...