Unknown Error
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.
Note: You will only see these details if you are saving Flow data along with Flow history (which is on by default for new Flows). If it’s not on, you can turn it on using the “Save data” checkbox at the top of the Flow History pane. Then the next time you get an “Unknown error” you’ll have more information to try to track down the cause.