Get "File mode specification error: (error Invalid pattern)" when opening XML file with RNC schema in nxml

[Steinar Bang]

Platform: debian 11.5 "bullseye" on amd64
          nxml installed with GNU Emacs 27.1

When I open an XML file with an RNC schema in emacs, the following
happens:
 1. The window is split in two, with the XML doc on top and the schema
    in the bottom half
 2. The cursor is positioned in the RNC schema
 3. The following error text is displayed in the minibuffer:
     File mode specification error: (error Invalid pattern)

See screenshot: 
https://www.bang.priv.no/screenshots/Screenshot%20at%202022-11-13%2010-52-49.png

In the messages buffer, I find this:
Error: (error "Invalid pattern at position 28615 in 
~/workspaces/oldalbum/oldalbum.db.liquibase/src/main/resources/oldalbum/dbchangelog-3.9.rnc")
File mode specification error: (error Invalid pattern)

Does anyone know how to debug this further?

The RNC schema, the XML file and the schemas.xml file can be found here:
 
https://github.com/steinarb/oldalbum/tree/master/oldalbum.db.liquibase/src/main/resources/oldalbum

The dbchangelog-3.9.rnc file was created by first either running the
(increasingly hard to run) sun schema converter, or maybe this one
http://debeissat.nicolas.free.fr/XSDtoRNG.php

and then converted from that to RNC.

And then I have done a manual massaging to make nxml accept the RNC
schema:
 
https://github.com/steinarb/oldalbum/commit/af4106c09b9a93b07d44f42348bab4b9f3ecaa0f

But now, two years on, the schema no longer works.

And the Relax-NG resources on the net seems to be dead, or nearly dead,
so I'm asking for help here.

Thanks!


- Steinar