I contacted Microsoft about an hour or so ago and this is the situation for 13-14 July 2013 regarding error code c101b000.
Looks like everyone around the world is having the same problem today.
There is currently an outage with our Marketplace Services that we are investigating at this time. Thank you for your patience as we work on resolving this as soon as possible. For more information on the service outage, please visit this page:
: http://support.xbox.com/en-US/xbox-live-status
I'm not too sure as to why it is on Xbox.com, sadly that is something left to the web developers at Microsoft. The Phone's marketplace uses the Xbox Live service as a gateway, I think they just grouped everything together on Xbox.com because of that.
Following on from that please go to the following link and vote for this Windows Phone Suggestion:
Make Error Codes On Windows Phones An Active Link To Report Problems
Make Error Codes on Windows Phones an active link, so when there are problems (eg error code: c101b000 - problems installing mobile phone aps) users can press/click on the error code and a message will go directly to the Windows Development Team alerting them that there is a software problem. It would also be good if after Microsoft were alerted to the problem they automatically returned a message to the sender with what the code meant and any other info (eg Microsoft were aware of a problem and that they would confirm when things were OK again). On the weekend of 13th July 2013 thousands of people around the world had the same problem with error code c101b000 and some complained on the forum site, but no-one knew why and were suggesting all sorts of things (eg turning wi fi off and on, taking out the mobile phone battery etc - none of which would have solved the problem). No-one knew what was going on and it took Windows some time to find out - by then thousands of people had been unable to install/update aps for a couple of days.