Given the little information provided, there seems to be an error when calling a webservice. Check your microflow for a webservice call and see if all parameters or path is correct.
If possible, give us more information to try help you on that problem.
As mentioned above, since there’s not enough information, everybody will tell you to both the microflow and the webservice in order to find where is the problem.
As everyone else was saying, the best way to troubleshoot issues like this are to get the stack trace. On your rest call activity right click it and click set error handling.
choose custom with rollback and draw a line out of the activity (right click the new line to set it as the error handling flow).
Now you can add a log message when there is an error.
There are a couple different tokens that can help here. LatestError will have the stacktrace and LatestHttpResponse will have the response from the server you are contacting. I would add both to your log message to help troubleshoot. If the LatestHttpResponse is empty then its a possibility that the request to the other server never went out and its an issue in your microflow. If there is a LatestHttpResponse, then check the content attribute to see what the server replied with.
Hope this helps!