Scenario: Trying to log into the web interface (HTTPS) of an APC PDU throws up a Someone is currently logged into the APC Management Web Server. Please try again later error. I suspect it was me when my browser crashed and now no-one else can log in.
To get around this problem either telnet or SSH to the PDU (I turned the insecure telnet feature off, in favour of SSH. The simply press number 4 to logout.
This will log the user out and you should then be able to access via the web browser again.