Citrix Error 51

Connection to server IP failed. Error: 51
When accessing Citrix Presenation Server using the Web Interface.
This occurs when accessing from a Macintosh client.

Solution:

On the client device, open the ICA Client Editor, go to the Options, Default Settings menu, and change the Network Protocol from TCP/IP + HTTP to TCP/IP.

That will work for most situations - including mine. That was until I made a change to our Citrix Presentation Server 4.5 farm and the problem started back up for our Mac customers.

I had enabled Asynchronous Writes (which works wonders on Windows clients) and it caused the Citrix Error 51 problem to come back. I did the following:

1) On the main policy, configure Asynchronous writes to Configured.
2) Create a new policy for our Mac users with Asynchronous Writes disabled. This new policy was given a higher priority.

The setting for Asynchronous writes can be found in Citrix Presentation Server Console under Policies > your policy name > Client Devices > Drives > Optimize > Asynchronous writes.