1 year later and I have just gone through this exact same problem again.
This time I think it was after I went through the process of renewing certificates etc.
It could be that this was the cause the first time? - looking through my records I had renewed my certificates on 5th June 21 - 5 days before my problems in post #1.
Am I doing something wrong when renewing certificates?
Don't say it. It is possible that you had more certificates in the past and it happened that you built the bridge for some other reason (new device for example) and therefore never encountered this case.