Documents fail with an error when they include \startelement[tagname]. This started on or before the 2020.11.17T12:42 LMTX update, and continues with 2020.11.18 19:16 LMTX. MkIV continues to work as expected.

Example:

\setelementbackendtag[myTag]
\setelementnature[myTag][mixed]

\starttext

 \startelement[myTag]                     % <---

  This works with MkIV but fails with LMTX, complaining: {\tt
  tex error on line 17 in file G:/extract.mkvi: The file ended
  when scanning an argument.}

  It works in LMTX when marked lines are removed, but\unknown

 \stopelement                              % <---

\stoptext

--
Rik