It took a while, but I did find out what changed. It's the initial part of the XML which causes problems. The culprit is the second line. If you change that back to the old version, the site prints again. It probably doesn't recognize it as a character sheet any more. You'll have to do this in each export.
I had some other errors at creation of the sheet, but those disappeared by magic. My mending cantrip must have fixed it by accident.
Old
<?xml version="1.0" encoding="iso-8859-1"?>
<root version="3.3" release="8|CoreRPG:4">
New
<?xml version="1.0" encoding="utf-8"?>
<root version="4" dataversion="20201016" release="8.1|CoreRPG:4.1">
Something you might not know: if you create an account, you can upload character images. If you select a character, choose "Large Font", that image will be printed on the sheet.