When you attempt to access your APC Environmental Monitoring Appliance via a web interface you receive the error "Someone is currently logged into the APC Management Web Server. Please try again later"
After confirming that no one is actually logged in it looks as though the web interface has got stuck. If near you could physically power cycle the unit to refresh the web interface. If the appliance is located some distance away the workaround is to telnet to it and then log off by selecting option 4 in the Control Console
Try accessing the web interface now and you should be able to log in as normal.
MartCJ 
I had the same problem and sorted by using FTP, open event.txt, see the last login IP address, and use logout.htm command from that IP address.
Telnet to the UPS IP. Set session timeout for the admin user by using ‘user’ command and -t or -st parameter (depending on your UPS version). That way all session will disconnect after the time you specify and you should be able to login.