Tribe Commerce PayPal Notifications stopped working

I have been using Event Tickets’ Tribe Commerce PayPal solution to sell event tickets successfully for the last year.

Since Aug 17, 2021, no new ticket sales are added to the event’s Attendees page. All tickets sold up to that point had been processed normally.

On Aug 17, I was running The Events Calendar 5.8.1, Event Tickets 5.1.6.
On Aug 18, before becoming aware of the issue, I upgraded to TEC 5.8.2, ET 5.1.7. The issue persists.
WordPress is at 5.8

Looking at Instant Payment Notification (IPN) history in PayPal, I see that all transacations prior to Aug 17 have status of Sent and all since then have status of Resending. During this period I have made no changes to PayPal, to the plug-in setups, or to the event itself. What else could have impacted the communication from PayPal back to the Event Tickets listener? Is there a way to examine the information available in the PayPal log and use it to debug what is happening on the Event Tickets side?

BTW, I have read your response to Sky Shabatura’s ticket “Tribe Commerce: Tickets paid but not shown” last month, but I don’t accept that it should be necessary to switch from Tribe Commerce to Woo Commerce. If Tribe is not prepared to stand behind your current PayPal solution (Tribe Commerce), why offer it? And if you believe it is necessary to rewrite the free Tribe Commerce component, should the alternative not also be free? Besides, what faith should I place in a paid Tribe plug-in if the free one doesn’t work?
I run this website for a non-profit organization with limited funds.