My snap wouldn't load because Flask was failing to load. I think it was a network problem that caused Flask not to start correctly.
Now that it is working again I completely disabled my network connection and the Snap/Flask start up ok so I suspect it is not the lack of network that is causing the problem but the test for Infura (possibly Infura was down for a short period?), notice that the check for Infura failed in the logs.
flask-unresponsive-log.txt