I'm getting this whenever the mod tries to do a backup.
And dimension 12 doesn't exist yet, no folder, no mystcraft agedata, ...
2013-08-06 02:00:25 [iNFO] [forgebackup] Starting a new backup.
2013-08-06 02:00:25 [FINE] [forgebackup] Disabling saving...
2013-08-06 02:00:25 [FINE] [forgebackup] Forcing an updated save...
2013-08-06 02:00:25 [FINE] [forgebackup] Creating new backup of your world...
2013-08-06 02:00:25 [FINE] [forgebackup] Re-enabling saving...
2013-08-06 02:00:25 [iNFO] [sTDERR] Exception in thread "Timer-0" java.lang.IllegalArgumentException: Could not get provider type for dimension 12, does not exist
2013-08-06 02:00:25 [iNFO] [sTDERR] at net.minecraftforge.common.DimensionManager.getProviderType(DimensionManager.java:157)
2013-08-06 02:00:25 [iNFO] [sTDERR] at net.minecraftforge.common.DimensionManager.createProviderFor(DimensionManager.java:304)
2013-08-06 02:00:25 [iNFO] [sTDERR] at net.minecraft.world.WorldProvider.func_76570_a(WorldProvider.java:207)
2013-08-06 02:00:25 [iNFO] [sTDERR] at monoxide.forgebackup.backup.Backup.doBackup(Backup.java:123)
2013-08-06 02:00:25 [iNFO] [sTDERR] at monoxide.forgebackup.backup.Backup.run(Backup.java:47)
2013-08-06 02:00:25 [iNFO] [sTDERR] at monoxide.forgebackup.backup.BackupTask.processArchiveBackups(BackupTask.java:36)
2013-08-06 02:00:25 [iNFO] [sTDERR] at monoxide.forgebackup.backup.ArchiveBackupTask.run(ArchiveBackupTask.java:16)
2013-08-06 02:00:25 [iNFO] [sTDERR] at java.util.TimerThread.mainLoop(Timer.java:555)
2013-08-06 02:00:25 [iNFO] [sTDERR] at java.util.TimerThread.run(Timer.java:505)