I am having this problem as well... YES, it's error code c101b000.
I tried to download 4 apps from the Marketplace, but when they actually download, they all stop at exactly 50% and revert to a "Attention required, Tap here." message... When you tap, an error box pops up and says:
App Name
There is a problem completing your
request, Try again later.
Error code: c101b000
***retry*** ***close***
when I hit retry, it downloads 50% and reverts back to the same loop.... when I hit close, it closes the window......
Samsung
Focus2
AT&T
OS 7.10.8112.7
Zune ver 4.8.2345.0
Windows Vista
Thank you for your time!