You have a tag with an empty slug. Did you upgrade from an older version of Netbox? This used to occur in old versions of Netbox if the tag consisted entirely of non-ASCII unicode characters. It *shouldn't* happen now, but if you have a way to reproduce it, please raise a github ticket.
To find and fix the problem:
- go into postgres (in Ubuntu: "sudo -u postgres psql netbox")
- try these queries:
select * from extras_tag;
select * from extras_tag where slug='';
- to update, choose a new slug:
update extras_tag set slug='xxx' where slug='';