A few weeks ago, I upgraded roundcube to make it compliant with php 8.x. I did not notice at the time but this broke compose and settings. At the heart of the issue were database structural changes that the upgrade script did not address. To resolve this, I had to re-install with a clean database. It is fixed now, but anything in the database, settings, signature, etc, are lost so you will need to re-enter these things.