I've seen issues like this in the past, albeit it has been few and far between. What happens when you disable only the firewall? Is anything showing in the Traffic log?
Also, review this article and follow the steps to get it setup so if SEP is blocking it, it is logged to the Traffic log.
http://www.symantec.com/docs/TECH203497
Or try allowing the correct ports:
https://technet.microsoft.com/en-us/library/jj572986(v=ws.11).aspx
You can also run SymDiag on an affected client to see what the result is.