First you say you get the same error message ending in "TypeError: not enough arguments for format string" -- which was fixed, by the way.

Then you say you
did successfully add the iOSRA plugin to calibre, but you still cannot connect to the Content Server?
The iOSRA plugin does not add the content server, the content server is built into calibre.
The iOSRA plugin allows you to connect your iDevice as a
device in calibre, and use send-to-device from calibre to send books.
If you cannot find the Content Server from your iPad's browser, the most likely problem is that your networking is setup wonky.
Sometimes there will be a guest band you have connected to instead of the main one -- so while you still have internet access, you are technically on a different network.
It is also possible your router disables connected devices from seeing each other.