Here is where I would start, just my personal opinion.
-does the issue happen with any windows computer?
-if yes, do they use specific vpn client version? if not, then you can definitely be sure its something with mac
-do you see any logs on the firewall when this happens? If yes, what does it show?
-as a test, can you try change visitor mode on the gateway settings to something other than 443 and see what happens?
I mean, yes, you can open TAC case, but Im positive they will ask you for same, if not similar info, so you might as well get it beforehand 🙂