This is caused by the configuration of our Web host allowing the disk to fill up. We're told that this occurs when other users of the hosting provider use the site backup option in their control panel. We're also told that it won't be disabled, because it would be inconvenient for users not to be able to back up their sites.
Apparently it's not at all inconvenient that users can't use their site much of the time...
Anyway, rest assured that we'll be badgering the host incessantly about sorting this out, or they'll be losing our business before too long. In the mean time, please do keep letting us know when the error occurs, so that we can track how serious the problem is (although obviously it's better that not every member e-mails us: perhaps the folks in chat at the time could arrange for one person to send the e-mail). Keep the subject line "SQL Error" (change the '+' in the default subject to a space, if it's there) so we can filter the messages nicely.
Thanks for your patience, folks.
Posted in: CoN Site News