From the link given by Windows Phone Support in the last tweet:
"c101a006 or "Internal Server Error"
This error code appears when trying to download an app and is caused by incomplete information in your Microsoft account profile or that the Terms of Service for Xbox LIVE were not accepted.
To resolve this, sign in to your Microsoft account (formerly known as Windows Live ID), tap or click Edit personal info, make sure that your Birth date and Home Country/region are completed, and then tap or click Save.
Note If you see a message indicating that your password is weak, change it to something stronger. If you are prompted to add additional security information, add it.
Or, sign in to Xbox LIVE using your Microsoft account and follow any prompts to accept the Xbox LIVE Terms of Service."