Converting Nav 2015 database to 2016 - gives locked object error even though no objects are locked

I'm upgrading from Nav 2015 to 2016 and following the guide from MS from here. I followed all the steps and made sure that no objects were locked (by filtering the object designer by Locked = yes). I have now done this multiple times but every time I try to convert the database in the 2016 developer environment I get an error saying:

There are some objects locked in the database. You must unlock these objects before upgrading the database.

Any ideas?