This looks to me as an error in the LMTX distribution:

tex error       > tex error on line 6 in file /Users/hansm/TeX/context-lmtx/tex/texmf-context/tex/context/fonts/mkiv/type-imp-dejavu.mkiv: ! Missing } inserted

<inserted text> 
}
<to be read again> 
\aligntab 
l.6 fontclass\aligntab
                    \fontclass\cr

Because running under my old ConTeXtBeta does not aborts with the error.
system          > ConTeXt  ver: 2019.06.01 19:11 MKIV beta  fmt: 2019.6.4  int: english/english

I looked into that file but could not see what causes the error.

dr. Hans van der Meer