That error may be due to that the database has been restored from a different server and there is a mismatch between dbo in the database and the database owner on server level. That can be addressed by changing the database owner forth and back.

But  EXECITE AS OWNER is not going to work for you anyway, since user_id() will resolve to dbo and not the actual user. Use certificate signing instead.

The problem is not with the user that cannot access sysusers, but with all the others that have elevate privileges of some sort.