After updating to the active directory plugin version 2.5 it has started to report my domain as invalid. I get the error text '<domain> doesn't look like a valid domain name' when clicking the Test Domain button on the Global Security settings page.
I think this is not working due to this commit and line linked in the commit https://github.com/jenkinsci/active-directory-plugin/commit/4e46466#diff-89b7b6ee37e6be852370249c03e7ca8cR283. It's checking for not null and reporting an error when it should do the opposite, at least from my read of the changed code.
If I visit the new Active Directory Health Status management page, it reports the following information:
DNS resolution : Domain was correctly resolved to NS/A record
Global Catalog : Global Catalog not exposed
Ldap Catalog : Ldap Catalog correctly exposed