It's a little late for this answer but still will give it a try. This seems to be an Odoo unsolved issue (happened to me with Odoo 12). There are a few workarounds:
quick fix: DELETE FROM ir_attachment WHERE url LIKE '/web/content/%';
If it doesn't work, try cleaning browser cache and retry.
If it doesn't work, just delete the db and postgresql's user and recreate them.
Recreating the DB usually works.
I know all of these are workarounds, but so far, this issue hasn't been solved by the Odoo team.
You can track the issue on GitHub