As revealed in the comments, this happens when a Meld window is already open. In that case, running meld again tries to use the existing Meld process; but that process can’t access the /dev/fd files which are used for the substitution...

There doesn’t seem to be an option to force Meld to use the “new” process, ignoring all others.

This happens when a Meld window is already open. In that case, running meld again tries to use the existing Meld process; but that process can’t access the /dev/fd files which are used for the substitution...

There doesn’t seem to be an option to force Meld to use the “new” process, ignoring all others.