MobileMe works on web site but not on computer - UNKNOWN VALIDATION ERROR

Laptop OS: Mac OS X 10.5.8

Hello. One of our users has a mobileme account which works fine on When clicking MobileMe in Sys Preferences on the laptop, the sign in keeps rejecting the credentials. We keep getting the error message: UNKNOWN VALIDATION ERROR: Your MobileMe name or password may be invalid. Please enter a different username or password.
Because of this, we can't sync the user's mobileme data to his laptop. The problem is specific to the users laptop as we have another workbook (OS x 10.4) which syncs mobileme fine.

So far we have done the following as part of our troubleshooting:
-Combined Mac OS X 10.5.8 update
-Software update for all other apps
-Deleted out all login keychains
-Verified disk permisions and disk using disk utility (both fine)
-Reset network settings back to defaults (renamed /Library/Preferences/SystemConfiguration/preferences.plist and rebooted)
-Tried using both our corporate internet connection via proxy server and dial up usb modem
-Created another Admin account but still get the same error
-Enabled all the protocals for our proxy server in Network Preferences for Airport

All this seems to point to an issue with (I'm guessing):
-Corrupted Keychain items
-Some setting in Security or File Sharing that's stoping the laptop contacting server
-Some setting with our proxy server which is fine for 10.4 clients but not 10.5
-Some setting on our firewall that's causing this.

We've tried all this plus a lengthy call with Mobile Me support. If anyone has any solution or troubleshooting suggestions we would be grateful. Please let me know if you need any other info.
Who is Participating?
bamford_supConnect With a Mentor Author Commented:
Thanks Strung. I'll give those a go and let you know.
bamford_supAuthor Commented:
Hi. Can you please confirm that 500 points went to Strung? Think I checked wrong box, sorry.
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