Did you have to recreate master with rebuildm.exe and then restore master from backup, at any point along the way?

We just had this exact problem as a result of a corrupted volume that required us to recreate and restore master. @@servername was null. select * from master..sysservers did not show the local server as a row with srvid = 0 - it was another srvid row thus @@servername was not finding it. This was on SQL 2000 SP3a and is a cluster instance.

We ran sp_dropserver on the local instance name (sp_dropserver 'server\server'). The extra row in sysservers was removed. We ran sp_addserver 'server\server', 'local'. This added the srvid = 0 row back to master..sysservers. (Aside - I have no idea what would happen if you did not sp_dropserver and instead did sp_addserver 'server\server', 'local',  'duplicate_OK').

You have to restart the SQL instance to get @@servername back.

Strange replication errors went away. One of the errors you get with this is

"Error 14114: (NULL) is not configured as a distributor" http://support.microsoft.com/kb/302223/en-us gives the @@servername clue. I suspect EM uses replication stored procedures, and these stored procedures are relying on @@servername having a value. sp_helpdistributor for example uses @@servername all over the place. We had the subject error too. I bet it's looking for MSdistpublishers on a server named 'null'.

I had this same thing happen on another server where we had to rebuild master, about a year ago. @@servername = null on that one too. I suspect there is some glitch in the master rebuild / restore / restart process.

Hope this post helps somebody else, because I've scratched my head on this twice now. It is difficult to find this solution.