Can the error be related to this?
From the doc I know that a storage volume can only be attached to a single app. But in this case I have a single app with 2 processes.
Given the way multi-process Fly apps work (each process gets its own VM on possibly different hosts), I don’t think the two processes can mount the same Fly volume. You’d have either use multi-process Machines or supervise multiple processes within a single Fly app, yourself.