usermanager suddenly stop working on port 80

Hi there,

we have couple of rb433 installed on various customer places.
today we receive complaint from two locations that said usermanager is not working.
we try couple tricks such us uninstall and reinstall user-manager but it does not help; user manager still can not be loaded.

usermanager start to work when the www port changed from 80 to 81 but have to add :81
before : 192.168.1.1/userman
now : 192.168.1.1:81/userman

The ROS used is v4.17

what is the possibilities of this problem ?

TIA

Paul