It just happened again, I lost several songs, I'd say the 4 that were in cache, then the next submitted.
EDIT : I didn't lost the next submitted track, it shows on Last.fm now.
Thanks for the log. What bothers me is that there's this...
2006-07-24 17:54:46 [LIBSCROBBLER] Preparing submit string - package size: 4
...which fails with "bad authorization", but then you closed XMPlay, reopened and...
2006-07-24 18:13:42 [CACHE] Number of entries in cache: 0
...so these 4 not submitted tracks haven't been written to the cache file (or have you deleted cache file between stopping and starting XMPlay?). I have an idea how to fix these "bad authorization" errors, but that's not the total solution, because even with such errors cached submissions should be handled properly - every track should get submitted at some point. I'll take a closer look at it and release a version with more debugging info produced in the log so I can gather more information about your situation... for now I can only say - sorry for inconvenience...