Happens on Debian 9 i686, amd64 and raspbian 9:
If the window is open and the client looses connection, the 
stays. Only if the window is closed, the “!” occurs.
These are all packages from the debian backports compiled by @schlarbm.
Can anyone test it if it’s also happening on other devices?
Edit: Appears also on Windows
I think it is a known problem that refreshing of the Applet Main Window sometimes lags behind.
I see that regularly when I change some files in a library. They get uploaded, but the last-changed entry is not updated until I click refresh.
I think these are different things. The tray icon is updated immediately, but the library list seems to be only updated from time to time.
Exactly.
That’s not what I ment (or what I understood). The libary window is updated with the “!” immedeately, but the icon keeps the 
. Only if the window is closed, the icon is updated.