I have been using the user manager over 2 Years now and for some reason I can no longer log in, everything seems to be working just can not log in.
If I login using putty on on port 22 then go to /tool user-manager customer print , it shows I am using the correct login name and password.
I have tried from different machines using different browsers it still reports login name or password incorrect.
Did you upgrade your routerboard of recent?
Try to reset the password and see if you still have the same issues
Same problem for me. After an upgrade to 6.7 all is working again 
No I have not done any upgrades, but last weekend, low and behold it let me log in, you can see from the post how long it has been out of action.
I have no explanation everything looks fine, have updated the users to match my Dalaradius entries on the main radius server, I only use the user manager as a backup if the main radius is down.