I got it working by uninstalling and reinstalling it, unfortunately, thus losing a day and half of messages.
FYI I ended up making a zip file of the /Android/media/com.whatsapp/WhatsApp folder, exporting it onto my computer, uninstalling WhatsApp (from the specific user profile) and then reinstalling it on the same user profile. It was the exact same .apk file that I had updated WhatsApp with earlier but wasn't working. Then, before opening WhatsApp, I copied back from my computer the .zip file and extracted the contents to where I had taken them from. I signed up again for WhatsApp with the same number, it detected the backup and I got back my account. Thanks to everyone for their help and suggestions. Something changed in Graphene, I believe, because AppBlock didn't get any updates, it just stopped working after the latest OS update.