I posted a blog on a few reasons why a PWA might go read-only - that main new one I was highlighting was for those customers who only have a Project Online Essentials license(s) which isn't enough to keep the site alive and it will get de-provisioned. We now male sure that the site will go read only - so that you know something is wrong and can get the correct licenses (add a Project Online Professional and/or Premium license) and then the site will come out of read-only. Full details at https://lwal.me/5g
Best regards,
Brian
