When I send a private message, I get the error. Message is being sent, but you don't see it until refreshed.
When you edit a post and go to more Options (f.e. to upload an attachment) it doesn't go further just displays the error.
And since I had to do exactly what I DIDN'T want to do, changing my ****ing nameservers over to CloudFlare's, nothing helps to stop this, not even deactivating CloudFlare (!!!)
Update: Flexile Theme is not compatible with CloudFlare. Generates tons of "Unspecified Error" Messages and almost nothing works. Standard theme still works though so it IS Flexile (I run dark standalone)
SOLVED! A simple uninstall of Flexile (Dark Standalone) and (re-) install brought the solution. Why changing your nameservers to CloudFlare would cause this, remains a mystery ...