As of 9pm last night, Authorize.net made a change to their API that effected MIVA merchant users. Neither MIVA nor Authorize.net has released an official statement, but an unofficial statement from MIVA said it had to do with Authorize changing a field to require case sensitive entries, and they rolled out the change thinking it wouldn't affect anyone.
As of 2 PM CST, Miva has released an update module to fix this problem, if your parent hosting company is aware of the problem, they have more than likely fixed it already. If you need to apply the update yourself and you have a MIVA 5.x store, you can obtain it by going Here
a MIVA 4.x update is still in the works, but is expected very soon.
Just to add, I can confirm that the update does indeed work, we ran the module across all 5 of our stores, and it worked flawlessly.
Edited by Uraiser, 25 February 2009 - 04:19 PM.