The following MWE

\enableregime[utf-8]
\mainlanguage[en]

\starttext
\input knuth
 \stoptext

crashes with the report

tex error       > error on line 2 in file
/Users/bowen1/Desktop/TestTeX/test.tex: ! LuaTeX fatal error error in error
handling


The culprit seems to be
\enableregime[utf-8]


Alan