I had the same problem with both mods - the error as above. However, the problem was eventually tracked down to a combination of incorrect test url (should be https://certification.authorize.net/gateway/transact.dll if you don't have an A.net developer account), transaction ID and MD5 hash. The MD5 hash entry field in A.net account manager is shorter than most MD5 hash generators create, so I, at least, had a mismatch in the string i'd input into the contribution and the string A.net was looking for. It was off by about 8 chars.
Double check your data and url in the first instance. The RC2a version now works fine with my MS2.2 store.
KB