Had that happen on a flight from CGN to LHR. What worked for me was to use the old check-in website. You can force ba.com to send you to the old check-in interface by making sure you're logged into any part of the old website frontend (e.g. https://www.britishairways.com/trave...club/_gf/en_gb - booking a redemption flight)

Note that the old check-in portal has its own quirks/bugs - iirc the "Save boarding pass" option gives you PDFs that are missing all of their images including the barcode (crucially). You'll probably need to choose "Print boarding pass" and then turn that into a PDF with your browser instead.