501 Errors - Linked to Proxy Management Issue?

Hi,
 
I've got a problem whereby some machines at a site are getting the dreaded 501 error.
 
One thing that I've found to be present on such a machine is that the Proxy Management Point location is different.  On a machine that gets the 501 error, it retrieves a Proxy Management Point from a different secondary site server!  However, on a machine that does not get the 501 error, it retrieves the PMP from the Primary Site Server (the only MP in the heirarchy).
 
Our setup is like this:
 
1 x Primary Site (Management Point) = PRISITE.
20 x Secondary Sites (Distribution Points only) = SECSITE1, SECSITE2, SECSITE3, etc.....
 
The problem machines are at a site called SECSITE1.  They are picking up a PMP from SECSITE2.  However, at SECSITE2, the machines are picking up a PMP of the Primary Site Server (PRISITE).
 
On machines that ARE working, there's a line in the LocationServices.log that says:
"Retrieving management points from MP PRISITE over HTTP."
then
"Retrieved proxy Management Point from AD: PRISITE"
I never see this line in the LocationServices.log of a problem machine.
 
Does this meant that there's a HTTP problem?  How would I check this?  Is it linked to IIS?
 
I'd appreciate any help you might have as this is really frustrating.
 
Thanks,
Peter

Yes, I know this is an old post, I’m trying to clean up older posts that are unanswered.
Did you ever figure this out?