Actually, I trying different combinations, I managed to get rid of the Members Filter bar altogether, even though it is ticked as visible.
The Registry settings for the Members Filter bar are:
Computer\HKEY_CURRENT_USER\Software\Raabe Software\MMX\15.0\Toolbars
"MMExplorerForm.MembersFilterbar"="FilterRegion=0,FilterEventHandler=0,FilterMethod=0,FilterComponent=0,FilterTypes=0,FilterShowAllMembers=0,FilterResClause=0,FilterVisibilityPublished=0,FilterField=0,FilterUnhandledTypes=0,FilterEvent=0,FilterVisibilityPublic=0,FilterLocalProc=0,FilterVisibilityPrivate=0,MemberSearchPanel=0,FilterControl=0,FilterAccessor=0,FilterVisibilityProtected=0,FilterProperty=0,FilterContainer=0,FilterVisibilityDefault=0"
"MMExplorerForm.MembersFilterbar#version"="7"