I just re-installed a fresh instance of v1.18.1 and latest LDAP plugin, configured the LDAP plugin, and was able to login as Agents AND Users with no issues. Something weird is going on in your setup or server. You really need to look at the AD logs on the AD server to see if an auth attempt is even being made for Users. You should also ensure that your LDAP plugin has both Agent and User authentication enabled. Sometimes you enable and save it doesn't actually save. Sometimes you have to clear all cookies/cache, go back to the LDAP plugin config, re-enable the settings, and re-save the changes.
Cheers.