I had issues with 7.6 on one RB4011 where all of the ports on one switch would randomly drop, and both switches would alternatively fail. I'd have to reboot it to get the failed ports to come back up. I submitted a ticket but got no response. I backed it off to 7.4.1 and have had no problems.
I had another one running 7.6, but I didn't notice the switches dropping on that one (the second one could have; ports 6-10 weren't in use). For consistency, I backed it off to 7.4.1 as well and haven't had any issues.
I'm a little gun-shy on trying 7.7 on either one.
Next time, I'll upgrade to 7.4.1 then.