I don't know if this helps, but I had a similar problem yesterday, and noticed that firefox was blocking a popup. I added an exception for app.roll20.net (in settings under Permission and Data, other browsers likely do it differently) and when I loaded the game, a popup appeared for about ten seconds that was logging into demiplane. This may or may not have been the cause (the problem seemed erratic, and the fact that it's gone away now doesn't mean that was the cause). But it's something to check if your browser is configured to block popups.