I had similar problems with upgrading foreman to 3.2.1 (Foreman 3.2.1 upgrade - The single-table inheritance mechanism failed to locate the subclass: 'Setting::Auth' - #2 by chr1s). In my case this was not only “Setting::General”, there were a lot more.
Perhaps some of the migrations did not work proberly? Also i noticed, that after correcting these values, something changed them back.