HI,
Today I have the situation which causes the problem. I have internet connection, but access to the backup is not available due to a network issue (a VPN connection is down)
When I click on the Enpass icon to start the program, the window opens but remains blank. Just a frame with title, and all white inside. The mouse pointer is a small blue rotating circle.
It remains like this for about 3 minutes then finally proceeds to the "Master password" prompt. After logging in the program opens fine and shows the available entries.
Checking the sync status shows, as expected, "Sync Error, please check your internet connection and try again".
So it seems that when there is no Internet connection the program skips syncing and goes on to open the program, but when Internet connection is present and the sync server is unavailable you have a rather long timeout before giving up and asking for Master password.
I've looked for a recent log file but so far haven't found one.