FWIW one of my Libraries (Intake) is on another computer. When I start Calibre, that computer WAKES, even though Main is the active Library.
Maybe you have a similar configuration BUT the computer is hard OFF or missing from your network.
Look at the switch List (right click the Library icon). You can 'Delete' that one (it only removes the switch link in Calibre)
Hmm

I wonder what happens if you have a
Connect To Folder (device) connection (on the Connet share icon)that was not previously ejected and the target is not ready. Is this a timeout wait?