Have now managed to get Download credentials to work (was using the wrong path, DUH...) but still the Replication credentials aren't working, even with Domain Admin account.
Managed to sort out, problem with user not product
OK, so you were able to sort it out . . . how? I haven't gone to drastic measures such as upgrading to 4.5, but I've got one server out of 85 that all of a sudden is doing exactly what you have described in your post. I can get download creds. to verify with no problem, but replication creds. fail every time. Have tried multiple domain admin accounts, deleted/recreated distributed repository in ePO, have deleted and recreated share on the server. I've noticed that even though it says invalid credentials, it doesn't lock the account in A/D.
How did you sort it out?
I ended up having to put everyone (read only) and domain admins (full control) in the security permissions as well as the share, re-ran the tests and it worked
Hope it's as simple for you...