NPC Sheets can't be closed

I can't close character sheets, even though the 'x' is still visible, and some of the other buttons - 'pop-up to separate page' and 'edit' - don't work as well, the only thing that happens is if you click them multiple times, the window gets resized to a bar, like with double-clicking on the title bar. It mostly happens when I change something in the token and avatar area, but also often happens for no visible reason. I work on Win 10, Firefox. Tried disabling extensions (only Privacy Badger and uBlock Origins, they return only blocking google analytics on the site, I have everything else enabled), or working on a portable version, had the same results. The only way to get rid of the character window is to refresh the page...

One of my player is experiencing the same issue for no reason whatsoever. Other players have also mentionned it but it's very inconsistent. I asked them about what extensions they are using but they only use adblocking extensions and all use Chrome, although the same issue is observed with Firefox.