I'm glad it isn't just me then
I was a little worried it may be to do wit hthe HTTP/1.1 503 Service Unavailable as well, so kept testing and it still did it when the track was successfully submitted. Observe:
2006-05-30 19:11:20 [DEBUG] DSP_Process -- sec <= 0
2006-05-30 19:11:20 [DEBUG] XMP_SubmitProc (bProcessed = 0)
2006-05-30 19:11:20 [DEBUG] XMP_InitDde
2006-05-30 19:11:20 [DEBUG] XMP_InitDde -- after DdeInitialize (instID = 603985028)
2006-05-30 19:11:20 [DEBUG] XMP_SubmitProc -- after XMP_InitDDE
2006-05-30 19:11:20 [DEBUG] XMP_GetChannelData
2006-05-30 19:11:20 [DEBUG] XMP_GetChannelData -- after DdeCreateStringHandle (instID = 603985028, hszService = 49153)
2006-05-30 19:11:20 [DEBUG] XMP_GetChannelData -- after DdeCreateStringHandle (instID = 603985028, hszTopic = 49154)
2006-05-30 19:11:20 [DEBUG] XMP_GetChannelData -- before DdeConnect (instID = 603985028, hszService = 49153, hszTopic = 49154)
2006-05-30 19:11:20 [DEBUG] DdeCallback (uType == 33010, uFmt = 0)
2006-05-30 19:11:20 [DEBUG] XMP_GetChannelData -- after DdeConnect (hconv = 637541764)
2006-05-30 19:11:20 [DEBUG] XMP_GetChannelData (id = 603985028, ch = info1, hconv = 637541764, hdata = 687874820, hszService = 49153, hszTopic = 49154)
2006-05-30 19:11:20 [DEBUG] DdeCallback (uType == 33010, uFmt = 0)
2006-05-30 19:11:20 [DEBUG] XMP_SubmitProc -- after XMP_GetChannelData (size = 836)
2006-05-30 19:11:20 [DEBUG] XMP_SubmitProc -- after XMP_FreeDDE
2006-05-30 19:11:20 [DEBUG] XMP_ParseTrackTags
2006-05-30 19:11:20 [DEBUG] XMP_SubmitProc -- after XMP_ParseTrackTags
2006-05-30 19:11:20 Submitting track: James Lavelle - Tribal Gathering '96 (Disc 3) (Tribal Gathering '96)...
2006-05-30 19:11:20 Submitting cache...
2006-05-30 19:11:20 ...
2006-05-30 19:11:21 Submission succeeded.
2006-05-30 19:11:21 Submit interval set to 1 second(s).
??!