[Volker]

Thank you for your response. You gave the right direction for further testing.

It seems, that the reason is outside OmniFocus. But currently I'm not sure if it is MissingSync or the synchronization with MobileMe. I've completed many syncs with OmniFocus, MobileMe and Missing Sync. But the error did appear only once:

At first I did an iCal-sync in OmniFocus, no problem. Then I did a sync in MissingSync for iCal Tasks and afterwards with MobileMe, no problem with both of them. But after some hours I initiated a sync with MissingSync again. Not tasks were change or added in OmniFocus, iCal and MobileMe. MissingSync indicated, that it deleted all tasks on the device (which device, I can't say) and wrote the same amount of task onto the device.

Before I started the iCal-sync in OmniFocus I checked the tasks in iCal. And as you said, the URL was deleted or changed. But I can't say, what was responsible for the change, which forced MissingSync to delete the tasks and rewrite them. Especially a lot of synchs were completed without the problem.

Nevertheless, after this check I did the iCal-sync in OmniFocus and voila, a lot of tasks (but not all!) were shifted to the inbox. After the correction, I'm now synching without the error for nearly 3 days.