Can you explain the issue in more detail? What plugin and what issue exactly?

TBH … I see no relationship between a plugin and the firmware of BBF Pro. The DAW creates the analog data which is finally being sent as a digital audio stream to the BBF Pro like any other audio data.

Where should there be a relation to the firmware?

You can ask RME support for a download link of older firmware.
But wouldn't it be better to find out the root cause?
If this is really a problem so that this could be fixed or not (like I assume at the moment).

This needs to be troubleshooted systematically.

It would be good if you provided additional information about your setup:
- Operating System / version
- DAW / version
- RME driver / version

I would use a fresh project, ensure that the proper RME driver is loaded.
What VST is not working, is it a VST or an VSTi? All or only a few?
Maybe you forgot to choose the output for the audio track?
Do you see any audio activity in TM FX in the middle row (software playbacks)?
Perhaps a total Reset of TotalMix FX (save your workspace to your PC before doing that, to be able to come back to this state quickly by loading the workspace if this was not the issue).

When I ran Samplitude, the popup windows said 'Failed to create VST effect: CLA-76 Mono' and plugings, etc. it seems all VST plugins were loaded failed.