Just installed DSM7 official release, no issues whatsoever. Had to upgrade the virtual machine manager and Plex, aside from that all docker containers include HA started just fine.
My issue is solved.
I had to add the DSM manually to the IP but without SSL checked and the non-ssl port configured.
Even though I don’t allow this access it is now working.
I’d thought I’d come back and update this.
A few days after my post this integration stopped working again - no config changes my side.
I removed the integration entirely.
I thought I’d give it another go a few days ago and now it just works again.
Must have been some kind of bug somewhere.