I am pleased to see that I am not the only one with this DLS problem.

Among the proposed solutions, there is only one that worked for me : create another user session.
But this is not very satisfactory for the workflow because my Logic Pro work sessions are on my main session. It makes you juggle from session to session. It's annoying.

To help understand all of this, I bring the following informations.
Under my main session, I still get the message "APPLE DLS ...". Therefore "Apple DLS" is never available in the midi input / output options.
However, if I first launch Logic Pro X (with an APPLE / DLSMUSIC / DEVICE instrument track), by setting the midi outputs of Band In A Box (option> midi input / output ports) to "BIAB Virtual Output" and selecting only the midi styles, I get the sound. But I don't have any sound at all if I select the Real Tracks styles.
And if I close LOGIC PRO X, the sound stops immediately in Band In A Box.

If I set the midi outputs (option> midi input / output ports) to "Midi Plugins", I have no sound, either for midi styles or for Real Tracks styles.

Does PGMUSIC at least work on these issues?

I was hoping the move to MacOs Big Sur would unlock something. But it is not.

I also tried to reinstall Band In A Box. But it didn't work either