Based on the URL you said is showing in the address bar when the problem occurs, it indicates that the hash that's regenerated using the digest/key isn't matching the expected hash and thus the validation of the transaction (on your store's end, to prevent fraudulent spoofing of approvals) is failing. Hence your store is refusing to complete the transaction.
Perhaps their tech support people need to do some specific debugging and troubleshooting in the code used in the after_process() function, on your store that's encountering these problems.