The iBoot is in use message is because the iBoot has only one socket and can only have one web connection open at a time.
If your unit is legitimately displaying the message, then it can be because another PC is logged into the unit, or you did not log out of the unit with the timeout set for 0. (no timeout)
This does not sound like your situation.
Some browsers had an issue that would create multiple connections to make the web experience faster.
Good for web surfing, but not the embedded devices.
There was another issue that occasionally would occur with the header size of the webpage as served within the TCP socket.
(some PC's were causing the header to be split across 2 packets)
When you get the in use message, can you refresh the page to get the setup page to appear?
I would recommend upgrading the unit to the latest revision, especially since you are going to use scheduling.
The time server section has been improved to include Daylight Savings and DNS for the time server address.
Upgrade information and files are available here: Revision History - iBoot-G2+
Feel free to contact Support directly should you need more information.