I've already spoken to my host, they said it is on default settings for mySQL
They lied. The default is 0, which is unlimited. Hosts set that variable to try and control use of their MySQL server.
And it seems like a "standard" is 20. So if there are 20 connections to MySQL (or 20 pageloads) at the same time, you'll get the error.
Do you run any other MySQL apps which might trigger this?
Nope, only the forum. The host it b-one, I'll try to look more into it and ask them again.