Going to subscription completion fails

When I submit a subscription, even if the payment is $0 the URL changes to ......?task=register.process_subscription but it takes me to a blank screen with 'AAAA' in the top corner.  The subscriptions had been working well so I have been working on other matters for some time, I have updated to Joomla 5.2.1 and MP4.1 but I don't know when this strange thing started occurring.  I may have to restore the site day by day until it no longer occurs.  However, I'd like to find another way to investigate this but I have no idea where to look.  I have checked the console and error-log, turned error reporting to maximum level but nothing shows up.  Can you suggest anything ?  See the private msg for login details.
 

This might help some.  I have looked through my subscriptions and my action logs.  on Nov. 19 at 15:47 I successfully created a subscription.  Immediately after that I raised my last topic with you - and there is a large set of updates you made - check in to #__fields, #__extensions.  After that I was out of town so did very little.  I didn't respond to your help.  It did seem to work very well and I have added a few more fields to the Joomla profile.  Other than that the main work I have been doing has been on Convert Forms.
 

Hello

Are you still having the issues here?

Tuan

Yes, no change. Have you found something? It still fails.

I have backed up to Nov. 20 but the problem was still there. I have then backed up to Nov. 17 and it is gone. Fortunately, I was the only one working in that time, apart from the previous topic you worked on, so I have been able to redo the work I missed. My server only keep 1 week of daily backups so it was good I had done an Akeeba backup only a couple of days earlier.
Plese consider this closed now. Trusting it will not reappear as I redo the past work.