I have a user that gets a message “An error has occurred. Please contact your System Administrator” when they try to block mail. They have version 8.5.2FP2. From reading posts about this it is a progamming error that was to be fixed in a later version. Was it ever fixed?
I had a similar issue that I resolved differently. You need to create a special view to display the mailrule documents. You will likely see a bunch of old rule docs in the view representing more rules than the user actually has displayed under the system Rules view. Delete these rogue documents and the error will go away.