I am having the same issues with v5.5 R2 hotfix 16 on a new test server (windows 2008 64 bit).
The first 2 sites were imports of existing sites from the dev server (windows 7 64 bit) and were fine.
The I create a new site (jntranet) on the test server (clean db schema) which appeared to work fine - can access the site, but it loops when I try to access CMSDESK. You can just see some crosses by the document names in the content tree.
This is in IE 8.
Thought it may be issues with database so changed connection string to dev database - same issue
Copied Dev site files to test so it should be an exact copy - same issue.
Changed browser to chrome - all happy! this looks like a weird OS / browser issue somehow.
On dev PC - windows 7 64 bit
Opera 10.60 - fine
Google Chrome 10.0.648.204 - fine
IE 8.0.7601.17514 - fine
firefox 3.6.10 - fine
safari 5.0 (7533.16) - fine
On test server - Server 2008 64 bit
IE 8.0.7601.17514 - loops
Google Chrome 10.0.648.204 - fine
Any thoughts on this odd behaviour?