Some time ago, I tried to buy an application from my Windows Phone 7 device and got an "Try again later" error message with code c101c401.
I also tried to buy the same application from my Zune desktop but it failed too, with no details this time.

After calling Zune support, I finally got the solution and I want to share it because the correction is simple and I could have done it if BING had found the solution for me :-)

In fact, a long time ago, I had renamed my default billing account on the billing.microsoft.com site as shown below :

This account has to be named "XBox Live" for Windows Phone purchases to succeed : just click the link below the combo box and you will be able to rename the account, with the setting below, everything is now fine :-)

Hope this helps...