After some investigation I found that SIGIL
checks every assigned drive letter for accessibility before showing the opening dialog. But it takes ages if one or more of such drive letters are disconnected and not accessible.

So the only workarounds I found so far are
- connecting all network connections
- remove all drive letters connected to a temporarily disconnected network
Any chance to change the source code to just open and not checking everything?
Kind regards,
Wasser Pulle