This works with the 20191028 LMTX beta, fails with the 20191028 MkIV beta:

    \starttext
    This\footnote{that}
    \stoptext

Error log:

    return {
      ["filename"]="c:/Users/micro/Desktop/fn.tex",
      ["lastcontext"]="\n<inserted text> ...s \\inheritmaintextcolor
    \\strc_itemgroups_push \n \n\\handlenoteinsert ...tes_synchronize
    \\the \\everybeforenoteinsert
    \n                                                  \\insert
    \\currentnoteinsert...\n\\235>notation:note ...enoteinsert
    \\currentnote \\currentnotenumber
    \n                                                  \\fi \\fi \\fi
    \\endgroup \\fi ...\n\\m_syst_action_nop ...dler
    \\currentconstructionhandler \\endcsname \n
    \\strc_constructions_finali...\nl.2
    This\\footnote{that}\n                      ",
      ["lastluaerror"]="?",
      ["lasttexerror"]="! Undefined control sequence",
      ["linenumber"]=2,
      ["offset"]=10,
    }

Can anyone reproduce this? I have tried rebuilding.

-- 
Rik