Hi @marrGuy
I too faced a similar issue. This usually happens if an audience definition is not correct.
In case of multiple audience:
1. Copy the activity
2. Select one audience at a time
3. Try saving the activity
This will give us a clarity for which particular audience this error is occurring and for that particular audience, we need to check its existence in audience library.
By doing this I was able to figure out the incorrect audience. Hope this will work for you as well.
Thanks