Can't Change Project Encryption Key

Running v1.2.3 in a docker container with persistence for the /data folder.

I was going to reset my project key, but can't (see below). Probably something stupid I'm doing wrong, but don't know what?

My project file currently has encryption:
image

I was going to reset the key, so I click edit and then the "trash can icon":
image

No matter what I type for the new key, when I click SAVE I get this message:

Any thoughts on what is going wrong? Not a lot to go by on the error message.