With the following example, I get:

    ...eXt/tex/texmf-context/tex/context/base/mkiv/grph-inc.lua:238: attempt to call upvalue 'imgwrap' (a nil value)

with the current (20190222) beta.

    \setupexternalfigures[location=default]
    \starttext
    \externalfigure[cow.pdf]
    \stoptext

Down for just me, or for everyone?

--
Rik