DTRegister 3.1.5c seemed to have fixed the issue we were seeing and all was good. Joomla had a critical upgrade that needed installing (3.4.6 I believe), so I installed it on my site. After that the issue cropped up again.
Today I was able to spend sometime on my site and after upgrading to Joomla 3.4.8 and discovering/installing DTRegister 3.16 today I'm still seeing the issue. I've had 2 Paypal successes and about 8 failures (at least one of those is verified as happening after the upgrade). I've cleared the cache in Joomla and the expired cache as well.
edit: just had another registration where paypal was processed, but no registration inserted thereafter
Unedited my original edit -- Registrations still not going through. Getting paypal emails saying funds accepted, however system not inserting records into sessions/event and folks are emailing me saying they haven't gotten the usual confirmation email.
Hoping this is a simple tweak in the code like last time.
Help! (Ticket filed as well if that's the preferred method of resolving the issue.)