InfoPath Dev

We have multiple forms that have been published and working for a number of years in our SP2013 farm. we're slowly working them out of SP2013/Infopath but for those we have not completed, we're starting to fight with behavior changes. The most notable is related to opening forms (new) to a system (after a form cache has been cleared, or a new user) where the template is shown as Access Denied. The specific error reads: Infopath cannot open a new form. The form cannot be opened. To fix this problem, contact the form designer. The Details show Form template: [location to file] Access is denied. we're trying to narrow this down in a way that allows us to understand where the error is coming from. we've tried moving the template/form to a local path and see similar behavior. The only things I can assume is involved is the shifts away from IE and use of IE mode with Edge, but I don't have proof this is the case. I've messed around with trusted locations and settings in the forms, but I'm not finding much to point me in any specific direction. I'm curious if anyone else is seeing/experience similar behavior or if you might have any suggestions on what to look at. Everything I've checked tells me it's not a permissions issue. But infopath continues to tell me access is denied. Appreciate any help you can offer.

Ick. I've seen errors like this before in designer, but haven't used a filler form in a long time. I tried to set up a test, but only have SharePoint online, which won't let me set to IE mode. You aren't using MFA on SharePoint 2013? I actually don't even know if that's an option.... I'm sure you've tried this, but what about some kind of network traffic tracing tool like WireShark to see if there is any better info?

I've done some packet traces (MS/wireshark/Fiddler) with limited value and nothing clearly indicating what's going on. the behaviour seems to occur before a full session is setup, so we were looking into handshake protocols but didn't get much in the way of details either. MFA is not setup in SP2013 so not used. The frustration around this is this is working until you clear the form cache. then it stops working. and prior to some point in the last few (maybe 6) months it was also working just fine too. I'll dig a little more on the traces and see if I'm able to find anything more there. MS has been less than helpful too. They blame Sharepoint even when we can demonstrate this as not a sharepoint problem. trying to open the file from the local filesystem results in the same access denied message. Appreciate the response!