We've had two instanced of this happening recently (although other subscriptions are working fine). We are running the latest Joomla4, CB, and CBSubs.In both cases PayPal shows payment completed. In both cases there is an entry in the history log"User Subscription record created with status R"and also an entry for Store Basket Payment with status NotinitiatedHere are the history log entries for one of the instances 

The Notifications page in CBSubs has no entry for this transaction, just for the one before and the one after.  The history log has no IPN, just "User subscription record created with status: R" and the plan ID is 5 (we've had successful transactions with this plan before and after this transaction).  The history log shows the event in CBSubs happened at 2023-01-27 00:40:10 about 4 minutes before Paypal recorded the payment as completed on January 26, 2023 at 4:43:53 PM PST.  

Here's what Paypal says about the transaction in general, at the URL you provided, note the transmission FAILED message, 16 retries (!), last attempt to transmit 2 days after the transaction:

"Instant Payment Notification (IPN) details
"Message ID6GN16901R0721662HDate/ time createdJanuary 26, 2023 at 4:43:58 PM PSTOriginal/ ResentOriginalLatest delivery attempt date/ TimeJanuary 29, 2023 at 2:20:52 PM PSTNotification URLhttps://www.cal-sailing.org/index.php?option=com_comprofiler&view=pluginclass&plugin=cbpaidsubscriptions&cbpgacctno=1&cbpbasket=20896&do=gw&result=notify&cbpid=cbp63d31d6bb2b69847497898&format=rawHTTP response code500Delivery statusFailedNo. of retries16Transaction ID 1CT84856NT557023R IPN typeTransaction made"

Here's a redacted copy of the IPN message from Paypal

"mc_gross=99.00&invoice=N12289825104673750&protection_eligibility=Eligible&payer_id=58Z77KHT9HU4J&payment_date=16%3A43%3A53+Jan+26%2C+2023+PST&payment_status=Completed&charset=windows-1252&first_name=REDACTED&mc_fee=3.95&notify_version=3.9&custom=20896&payer_status=verified&business=payments%40cal-sailing.org&quantity=1&verify_sign=Af3liDHWQ1jVkPN5H.CWpgxJiAYPAhGD-E3UITA4XNIPIdnDgcy49YrI&payer_email=REDACTED&txn_id=1CT84856NT557023R&payment_type=instant&last_name=REDACTED&receiver_email=REDACTED&payment_fee=3.95&shipping_discount=0.00&receiver_id=RLD8NFQH8WGEG&insurance_amount=0.00&txn_type=web_accept&item_name=Paid+Student/Senior+for+REDACTED&discount=0.00&mc_currency=USD&item_number=S20944-18822-5&residence_country=DE&shipping_method=Default&transaction_subject=&payment_gross=99.00&ipn_track_id=f209262e1e541"

This was a new purchase. We have not altered the DB or experienced any problems in other components that would indicate a general DB problem (we have 20 of our own components).
PHP error log has nothing for the date/time of either of the two problems. We have a shared server on SiteGround, and I can't get the Apache logs for that period.