SUSE Manager Upgrade fails from version 5.0 to 5.1

SUSE Manager 5.0 on SLE Micro 5.5
SUSE Multi Linux Manager 5.1 on SLE Micro 6.1

An attempt to upgrade a SUMA server from version 5.0 (running on SLE Micro 5.5) to version 5.1 (running on SLE Micro 6.1) results in database problems that prevent the services from starting correctly, and the SUMA WebUI becomes inaccessible. 

The issue occurs after the migration completes.

The following logs can be seen in the journalctl:

spacewalk-startup-helper[430]: Upgrade failed, please see log [/var/log/spacewalk/schema-upgrade/schema-from-20250829-090706-to-susemanager-schema-5.1.10.log].
spacewalk-startup-helper[430]: You can fix the failed migration in [/var/log/spacewalk/schema-upgrade/schema-from-20250829-090706]
spacewalk-startup-helper[430]: and run spacewalk-schema-upgrade again
spacewalk-startup-helper[338]: Database schema upgrade failed. Please check the logs.

And following log can be seen in the schema-upgrade log: 

                                              ?column?                                              
----------------------------------------------------------------------------------------------------
 susemanager-schema-5.1.7-to-susemanager-schema-5.1.8/200-proxy-server-group-members.sql.postgresql
(1 row)

psql:<stdin>:10: ERROR:  more than one row returned by a subquery used as an expression

Edit the following file on the affected server:

/usr/share/susemanager/db/schema-upgrade/susemanager-schema-5.1.-5.1.8/200-.sql.postgresql

Replace its content with the following SQL commands:

 INSERT INTO rhnservergroupmembers (server_id, server_group_id)
 SELECT rpi.server_id, sg.id
 FROM rhnproxyinfo rpi
 JOIN rhnServer s ON rpi.server_id = s.id
 JOIN rhnservergroup sg ON sg.name = 'Proxy'
 WHERE
     s.org_id = sg.org_id
     AND NOT EXISTS (
         SELECT 1
         FROM rhnservergroupmembers sgm
         WHERE sgm.server_id = rpi.server_id
           AND sgm.server_group_id = sg.id
     );
 UPDATE rhnservergroup sg
 SET current_members = (
     SELECT COUNT(*)
     FROM rhnservergroupmembers sgm
     WHERE sgm.server_group_id = sg.id
 )
 WHERE sg.name = 'Proxy'; 

Restart the services to re-trigger the migration:

spacewalk-service start

The SUMA upgrade process from version 5.0 to 5.1 fails due to a database migration issue. An SQL query within the schema upgrade process does not execute correctly, leading to inconsistencies in the database schema. This prevents the SUMA services from starting and makes the WebUI inaccessible.