Ongoing implementation of MBS-9902 as pull request #2184 submitted by y-young is adding validation of a set relationships to the same URL.

For example, it should be possible to link to Mainly Norfolk songs using "lyrics" relationship when appropriate (the editor still has to check it manually for now), but linking as "other database" is mandatory.

The initial error message is generic, but it would be more helpful to add an help link that shows all the allowed combinations of relationship types for that URL.