Problem establishing trust between domains

Question

Active Directory

There are two domains:
firma.d1 (legacy of the past) DC=server.firma.d1 (Win2008 R2)
firma.local (new domain) DC=n_server.firma.local (Win2008 R2)

Task: transfer all users from the old domain to the new one and computers without interfering with people's work.

DNS works fine in both domains. The forward lookup zone of the "other" domain is created as an additional zone in each domain.
The network is working fine, DCs of both domains see each other (by full name) and ping without problems.

When I try to set up a trust between domains, I get an error in the wizard:

Failed to create a trust relationship due to the following error:
The operation failed.
Error: This operation cannot be performed on the current domain.

The windows logs are empty.

1. What can be, where to dig, where to look?
2. Could it NOT interfere with the fact that the actual netbios domain names are identical - "firma"?

Answer 1

> Whether that actually netbios names of domains are identical "firma" can interfere?
Maybe.

Answer 2

It is worse to rename a new domain - there are already many users and machines under it, and there are much more chances to rake problems from this side.
I’ll rename the old one, check if it’s possible to get trust between domains, if yes, I’ll leave it like that, if not, I’ll roll back to backup.