I'm having the same issue which started occurring about the same time as you. I have a support ticket in with Avactis on this and they are trying to track down where the problem is occurring and why. Unfortunately, this is taking a long time to track down as the error does not occur for every credit card and seems very random on the ones that get the error.
I know this doesn't help right now, but there may be a fix for this soon.
Update: Well it seems that I was passing the product detail to Authorize.net when using AIM, and I had some special characters in the product detail descriptions. This combination of things was causing the error to occur. Interesting that the actual error had nothing to do with the error reported.
The fix for this was to turn off passing product detail to Authorize.net. You only need to pass product detail if you are using the SIM method.