I've posted before on how crucial Application Insights is to troubleshooting B2C custom policies.
Once you had got it setup, you need to wait about 5 minutes and then run something like:
This displays all the data. You can sort by clicking on the "timestamp column".
Expand the "message" section.
Expand the "FatalException" section.
and you'll see the error.
If you want to filter the errors, try something like:
| where severityLevel > 0 and message contains "Exception"