In our enviorment we use DS 6.9 SP4 and we manage MAC OSX 10.8.x with agent 2.6-92 (that comes with SP5).
We do not have any major issue.
The only case we opened with the support line is about the impossibility for 2.6-92 to recognize internal disk greater than 1 TB, they are actually reported in MB (e.g. 2 TB hard disk is reported as 2 MB).
This carries a failure for our job conditions that check for free space before installing a softwware.
Anyway, I know Symantec is working on SP6 that should be released this year.
Regarding your specific issue, i don't think is something with the agent but, probably on your system settings/configuration. Did you try to analyze traffic on a machine, to understand which communication and on what port is failing?
Regards,
Mr.Mik