A tiny but essential piece of code was missing...

modules/Bio/EnsEMBL/Registry.pm

@@ -298,6 +298,7 @@ sub load_all {
                             $adaptor, $section );
                 }
 
+                require($adaptor);
                 $adaptor->new(%adaptor_args);
 
             } ## end foreach my $section ( $cfg->Sections...