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