I received the same error message when installing multiple apps. The app downloads but when it goes to install (at about the 51% mark in the status bar), it errors out and goes to the App list. The app name is shown but under the name it says "attention required". Clicking the app shows the app in Marketplace Downloads with "Attention required. Tap here." Clicking gives "<app name> There is a problem completing your request. Try again later. Error code: c101b000" with retry and close buttons. Retry ends up giving the same behavior. This occurred even after a shutdown and restart of the phone.
However... After trying a few times, one of the apps did indeed install. I still have another which is giving the error but I'll keep trying.