The other day I ran into a problem with an old Windows 2000 Citrix Server which suddenly stopped accepting some connections. The problem turned out to be that they recently updated the server with Rollup Fix 1 (from 2005). It seems that there is a problem with RO1 together with Citrix. The fix needs to be manually downloaded from MS.
Links
http://support.microsoft.com/default.aspx?scid=kb;en-us;891861
http://support.microsoft.com/kb/904711
http://support.citrix.com/article/CTX107051