Thank you for people who sent the email notifications informing me of the error.
The entire site went offline for 1h 55 minutes today (starting about 11am) and it looked like some kind of overload - possibly an attack.
After that there was Data Corruption in the forums which screwed everything up and meant that the forum stopped working correctly and displayed frequent MySQL error messages. (so another ca. 2 hours).
Everything has now been fixed - with 900,000 posts it takes a while so thank you for your patience and lets of course hope that it is now resolved.