We can try a few things with secrets to see if that helps. Make sure you upgrade your flyctl first if you haven’t deployed recently.
Can you run fly secrets list and make sure everything looks like it did when you first set up the app?
You could also try resetting the secret and doing a redeploy (fly config save, thenfly secrets set NAMEOFSECRET, then fly deploy). Like this user here V2 Migration failed to migrate a secret correctly.