Another update: Everyone who was accidentally unenrolled is now re-enrolled. Apologies again for the mix-up.
When moodle.org was upgraded to 2.0, the enrolment method of all users was set to manual, rather than self-enrolment, resulting in users being listed as course participants forever, unlike users who self-enrolled since the upgrade, who are unenrolled after 2 months inactivity. Thus, the number of participants in Using Moodle is over 8000, when it should be closer to 4000, causing a big load on the server when forum post notifications are sent.
I was trying to run a script to convert manual to self enrolment in just one course, but obviously it went totally wrong so back to the drawing board with the fix and I promise to make sure that it's tested more thoroughly next time!
PS Howard receives the prize for being the first to inform me - very helpful, since I was also thrown out of Using Moodle, so didn't receive any notification of Joseph's post!