Can't add items/images to Token Marker Sets in Token Marker Library

I can't add items/images to any Token Marker Sets in the Token Marker Library. I've tried Chrome and Firefox, from a game, from the Tools menu, various .png images sized to 512x512. I've tried multiple sets, creating new sets, creating a new account to see if that was an issue. Nothing helps. They'll upload and presumably are fine, then after the Processing Files This might take a few minutes. Hold tight: we’ll update you when the process is finished. message is displayed, it shows a "Rename files" page that's empty, and won't save anything if I click Save.

Update: It seems it's only affecting images if at least one in the batch has a '+' character in the file name. I still can't see anything in the rename screens, but renaming the offending file lets me upload images, and then rename them afterward. (Still can't add a '+' character.)