Weird indeed. Exactly the same thing has just started happening on my online server (7.51a), but only on version 7 boards (I am still running some legacy version 6 ones). I think it may have done this in the past, but looks like a server reset cures it. The code hasn't been changed for months, and I'm sure it wasn't doing this earlier in the week.
The online server is the latest Win 2003 Server with IIS 6. The offline server has identical code, but is running Win 2000 and IIS 5, and doesn't have the problem.
Any ideas how to stop it doing this would be very welcome.