Do you have another interface with an ip assigned? The hotspot interface tends to block any html besides the hotspot stuff. Try using your public ip or another private ip on the router. http://192.168.88.1/userman
If you are trying this from the hotspot interface, you must be logged in.
I forgot I did an upgrade and had to reinstall user manager. I just tested it here and it works fine as long as you do not use the hotspot interface ip.
You do have it installed, correct? It is not installed by default, and must be re-installed after every RouterOS version upgrade.
When I install User-manager package, everything is OK.
I connect to Mikrotik via Hotspot, login with admin admin, then go to mikrotik Ip adress (not Hotspot) http://192.168.1.5/userman and I see User Manager.