Recently, I upgraded a Mac Pro From High Sierra to Mojave. The FMS version was 17.0.203 and External authentication was running perfectly. After upgrading to Mojave and upgrading the FMS to 188.8.131.52, External authentication has become intermittent. File-driven authentication works fine. But authentication against Active Directory groups has become hit or miss. It tends to work fine for me yet when I ask one of our users to attempt to log in, FMS tells him "the provided account name and password cannot be used to access the file."
The ONLY external account that seems to work is my own. However, a test user I created and placed in the same group will gets the denial message. It seems very random. I followed the upgrade paths to the letter. And again, local accounts, and my AD account seem to work.... but no one else's AD accounts work. Not even the test account.