It’s probably due to the / in the name.
If you want to manually remove the thing, you can SSH into your Raspberry Pi and do the following:
$ sqlite3 ~/.mozilla-iot/config/db.sqlite3
SQLite version 3.16.2 2017-01-06 16:32:41
Enter ".help" for usage hints.
sqlite> select id from things;
<snip>... some things </snip>
sqlite> delete from things where id = "<the faulty id>";
sqlite> .quit
After that, you can restart the gateway and things should be fine.