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