In my case an upgrade was prohibited because i had installed libapache2-mod-security, which interferes with the custom mod-security setup that plesk uses. I uninstalled mod security and upgraded my plesk install from 12.0.x to 12.5.x and now I don't see the error anymore.
Note: Solution likely to be different for people experiencing the same issue
2
Answers
In my case an upgrade was prohibited because i had installed libapache2-mod-security, which interferes with the custom mod-security setup that plesk uses. I uninstalled mod security and upgraded my plesk install from 12.0.x to 12.5.x and now I don't see the error anymore.
Note: Solution likely to be different for people experiencing the same issue
What error are you getting in plesk error logs file ? Also please try to update your plesk panel to latest stable version.