Are you using the VTT Enhancement Suite extension? I had this problem yesterday (and still do) but if I turn off VTTES then the character sheet opens ok. I suspect Roll20 has changed something recently that causes an issue with VTTES. I believe (looking at the VTTES Discord channel) that the VTTES developer is aware of the issue. He generally responds quickly to issues, but obviously it depends how much of a rewrite he needs to make to regain compatibility with any R20 changes. If the problem you are seeing does go away if you turn off the VTTES extension, then I doubt you'll get any help for it on here ;-) (edit) - and if you find it is a problem with VTTES, check for the latest update of that extension - the dev has today released an update which fixes this problem (for me at any rate)