5
My site is live for appr. 3 years now.
The problem didn't happen from the start, but I also don't know when it did start.
What I know is that people had the same problem/error when they tried to contact me by using the Contact module (version 1.6 from CreepingDeath - XOOPS@IBDeeming).
I've replaced it with Liaise and this works.
So I guess the problem is in XOOPS somewhere.