And finally the Longhorn manager logs:

kubectl -n longhorn-system logs <longhorn manager pod name> | grep <volume name>

If you want to have all relevant info together (log, configs, ...) you can generate a support bundle following this procedure: https://www.suse.com/es-es/support/kb/doc/?id=000020145

That's just a zip file with all the info available that you can unzip and use grep/find with the volume id to find all the references.

Longhorn KB has several articles covering attach issues that can help you or at least give you clues about how to better debug your specific issue: https://longhorn.io/kb/