How can I find out what triggers a completely white application window that is displayed during every Windows startup?

Since a little while every single time when I start Windows 11 a completely white application window appears and then disappears again after roughly 10 seconds. It doesn't even have a title bar. It's just a completely white square. Sometimes when I mouse over this white window a tool tip displays, but confusingly not always with the same text. One time it read "PDFgear", but then another time it read "Discord". Both of these applications are also not in the list of startup-apps in Windows Settings.

Does anyone here good suggestions for how I could investigate what exactly is triggering this white window in the first place? (because I'd like to get rid of it)

Is it like this: -