I've had frustating problems with error code c101b000 on my lumia 535 for a week before I fixed it this way:
SOLUTION:
1. Back up your phone contacts and files on another device.
2. Hard Reset your phone and when you're asked, click to wipe your sd card also. (first time around I only hard reset the phone in settings > about > reset phone without confirming to wipe the sd card and the error c101b00 wasn't fixed. I hard reset it again, but this time clicking to wipe the sd card and the error was fixed.)
3. After your phone resets, make sure your time and date is correct before downloading apps.
You should be able to download apps from windows store now. You're welcome
Why should you also wipe your sd card?
Because error c101b000 indicates a register (overwriting) problem. This problem usually occurs when old apps that you previously uninstalled, didn't uninstall properly and left traces of code on your sd card and phone storage. When you try to download new apps there's a struggle for the apps to be written over the incompletely removed apps.