Have a look at this document:

After migration to 11.0 RU7 clients are not updating or connecting - Sylink.log 500 internal server error

http://www.symantec.com/docs/TECH168828

HTH!

Hello,

Are you using Windows 2008 Server / Windows 2003 64 bit server?

Could you pull and upload the sylink.log from the newer clients  to us to check the root cause of this issue?

Secondly, are these newer client machines cloned / imaged ? If yes, check this: http://www.symantec.com/docs/TECH96808

Are you using any proxy?

Check this Article and work on the steps provided in it.

After migration to 11.0 RU7 clients are not updating or connecting - Sylink.log 500 internal server error

http://www.symantec.com/docs/TECH168828

OR / AND

Possible Causes: Legacy proxy settings in the registry still persist after environmental changes on client machine.

Solution

The legacy proxy settings can be removed by performing the following steps:

1.   Open the registry (Start->Run->type "regedit").

2.  Go to HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\InternetSettings\connections

3.  Delete the registry keys "DefaultConnectionSettings" and "SavedLegacySettings".

4.  Reboot the machine.

Note:  These registry keys will automatically regenerate after reboot of machine.

Also, this also could be caused due to incorrect proxy server information in the following registry location: HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\InternetSettings

Removing the incorrect proxy info from this key and then rebooting allowed the client to communicate normally.

One important thing to keep in mind is that any incorrect proxy information must also be removed from the following two locations as well:

HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Connections\DefaultConnectionSettings HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Internet Settings\Connections\SavedLegacySettings

If the settings are not removed from these two keys, they will repopulate the Internet Settings key after every reboot.

Remove the incorrect proxy information from all 3 registry locations noted above, then reboot.

Just to add, in many of my cases, where was an issue with the System Account at User Proxy Level. They had to Bypass the Proxy on the server.

Hope that helps!!

Try by disabling firewall and UAC

Try this as well

Replace the Serdef.dat file on the SEP 11 Client

1. Export a new package from the newly installed SEPM without checking the 'Single EXE' option
2. On the SEP 11 Client machine, click Start > Run and type 'smc -stop'
3. Copy the serdef.dat file from a working system and paste it over the existing version at \Program Files\Symantec\Symantec Endpoint Protection\
4. On the SEP 11 Client machine, click Start > Run and type 'smc -start'
5. The client should now successfully connect to the SEPM

Try by repairing SEPM....