Hi, i also have the same error message, right after clicking "Apply" when already selected storage plan and payment method. I tried 3 browsers, 2 computers/OS and i also tried paying for subscription via Windows 8.1 Preview settings and also got the same error message vie operating system message popup. Also tried the provided instruction in the beginning of thread.

Also with internet searching i found up a possible cause being that skydrive upgrade page was not in the same language as the billing account, and some people resolving the problem with Microsoft support giving them a manual link for their language/country, changing the "en-CA" at the end of this link, which didin't work for me(sl-SI):
https://skydrive.live.com/options/upgrade?accountid=0&ctpmkt=en-CA

Here is the MIcrosoft account in question if needed: *** Email address is removed for privacy ***