It sounds like you are using Backup Exec as a trial version, is that correct?
Since you did not have 6235 installed prior and have not verified that the issue you are receiving in 7170 is due to the same error which is resolved in HF 21 then I would suggest to call support and have them debug the agent on the Lotus server to ensure your issue is the same. There is not a Hotfix 21 or its equivalent for build 7170 at the moment.
If you are using Backup Exec as a trial version then I would get the download for 6235 and install all the necessary hotfixes (Especially HF21) and run your Lotus jobs using that. Then upgrade to 7170 when the equivalent HF is developed for 7170.
If you are using a licensed version of 11d 7170 then I would definitely contact support to debug the agent on the Lotus Server in order to verify this is the same issue as seen in 6235 and corrected by HF21.
- Jared