Hi David Galway,
Thank you for posting in the Microsoft Community Forums.
Check and adjust the Group Policy settings:
Disable the “Passwords must meet complexity requirements” policy (as a test only) and try changing the password.
If the problem is resolved, re-enable the policy and check that the other password policy settings are appropriate.
Force a Group Policy refresh and check DC synchronization:
Execute the gpupdate /force command on the client computer.
Check that the password policy settings are consistent on all domain controllers and that they are properly synchronized.
Check and repair user profiles:
Create a new profile for the user and check if the problem is resolved.
If the problem persists, check the ProfileList entry in the registry and try to repair or rebuild the registry entry.
Check for client computer and network problems:
Make sure that the operating system and all related software on the client computer have been updated to the latest version.
Check that the network connection is stable and make sure that the client computers can successfully connect to the domain controller.
Collect logs and error messages:
Collect the Event Viewer logs and any relevant error messages on the client computers.
Analyze the logs and error messages for more detailed information about the problem.
Best regards
Neuvi