What is strange is that after my three forums come back online, one of them displays a server error message, which makes the forum unusable. Repairing the database seemed to fix this.
The same thing happened on the second occasion. My forums went offline and when they came back online, one particular forum (the same one) had the server error message.
They have advised that my forums have been attacked by the IP 18.104.22.168. I checked an IP lookup service and it states that it is a known spam source.
As I upgraded my forums on the same day, I am not sure if this is a security vulnerability of my new host, of XenForo 1.1.5 (I was previously on 1.1.2), or if it is just a combination of coincidence and bad luck that this happened on the same day I moved host and upgraded.
Does anyone know what is causing this?