Can be a problem with uniqueMPgraphic because useMPgraphic works.
\startuseMPgraphic{wrong or right}
pickup pencircle scaled 3pt ;
path p ; p := unitsquare
xscaled OverlayWidth yscaled OverlayHeight ;
fill p withcolor .8white ;
draw p withcolor .625red ;
\stopuseMPgraphic
\defineoverlay[button][\useMPgraphic{wrong or right}]
\setupbuttons[background=button,frame=off]
\setupinteraction[state=start]
\starttext
\completecontent
\page
\hbox
{\button {previous} [previouspage]\quad
\button {next} [nextpage]\quad
\button {index} [index]\quad
\button {table of contents} [content]}
\completeindex
\stoptext
Wolfgang