UnknownError - Errors
An "Unknown Error" will result when a call to an application or other API returns an error with no message included.
A good way to get to the bottom of errors like this is to go to Flow History and click the execution that resulted in the unknown error. You should see a red error box on the card where the error occurred and inside that box will be the full details that were returned along with the error. Sometime, those details will include an error description and/or other details. That information plus knowing the card where the error occurred provide the first clues in how to track down the issue.