I got a problem with my DC configuration: most computers can access the GP just as they should, just one Windows Server 2003 R2 (SP2) is unable to read the policy information.
I get the following error in the event log: "Windows cannot access the file gpt.ini for GPO CN={31B2F340-016D-11D2-945F-00C04FB984F9},CN=Policies,CN=System,DC=Domain,DC=lan. The file must be present at the location <\Domain.lan\sysvol\Domain.lan\Policies{31B2F340-016D-11D2-945F-00C04FB984F9}\gpt.ini>. (The format of the specified network name is invalid. ). Group Policy processing aborted." (Source: Userenv, Event ID: 1058)
From this computer I can't access the share \Domain.lan\sysvol, while other computers can access it without problems.
I ran DCDiag and NetDiag on the DC, both without any errors.
What else can I do to find the source of this problem?
Sounds like a name resolution problem. You don't mention what kind of error message you see when you say you "can't access the share \Domain.lan\sysvol".
What does an "nslookup domain.lan" command run from a command prompt give you? You should get back a list of the IP addresses of your domain controller computers.
You've double-checked the DNS servers specified on this client to see that it's only referring to one or more domain controllers running DNS servers (i.e. no ISP DNS servers specified, etc)?