This appears to be similar to this problem:
The dialog box appears as stated in the above message after clicking "add" to try to add ACL entries.
This is repeatable on my 3.0.25rc3 print server in a Samba-controlled domain.
Trace file located at http://www.nanogherkin.com/smbdump.tcpdump.bz2
Configuration: 2 hosts, Samba PDC and Samba domain member.
I think that including the dns and forest name in the
DsGetPrimaryDomainRole() reply is wrong when configured for
security = domain.
Created attachment 2702 [details]
Patch to prevent a machine configured for security = domain to return a DNS and forest name in the DsGetPrimaryDomainRoleInfo() reply
I think this will take care of it. Please test the patch and reopen the bug if that does not fix it.
*** Bug 4965 has been marked as a duplicate of this bug. ***