The following code fails; however, it succeeds without newotf, and it succeeds with newotf if the defined font feature (*hebrew) is removed from the \definefont command. The characters that demonstrate the problem are HEBREW LETTER LAMED combined with HEBREW POINT HOLAM, followed by HEBREW LETTER ALEF (U05DC with combining U05B9 followed by U05D0). I have not come across other combinations that cause the problem, but I have not done an exhaustive search.
\usemodule         [newotf]
\setupdirections   [bidi=global]
\definefontfeature [hebrew] [default] [lang=heb,ccmp=yes,script=hebr]

\definefont        [Hebrew] [sileot*hebrew]
\starttext
  \startTEXpage
    This is okay:{\hfill\Hebrew לא}

    This is okay:{\hfill{\Hebrew לֹ}}

    This fails:{\hfill{\Hebrew לֹא}}
  \stopTEXpage
\stoptext
It reports:
error: C:/ConTeXt/tex/texmf-context/tex/context/base/font-ots.lua:1586: attempt to index a number value

tex error       > tex error on line 13 in file C://Users/rik/Desktop/newotf-features.tex: ?

<inserted text> \par

<to be read again>
\vss
\pack_framed_do_bottom ...amedparameter \c!bottom
                                                  \raggedbottomcommand
<to be read again> \localendstrut \pack_framed_do_bottom
                                      \egroup
\stopfittingpage ->\removeunwantedspaces \egroup
                                                 \egroup \doif {\fittingpage...
l.13   \stopTEXpage

The same problem occurs in the same way with the following fonts, most of which are shipped with Windows:
Arial Regular
Arial Bold
Arial Narrow
David Regular
David Bold
Ezra SIL Regular
Ezra SIL SR Regular
Keter YG Medium
Levenim MT Regular
Levenim MT Bold
Taamey David CLM
Tahoma Regular
Tahoma Bold
Times New Roman Regular
Times New Roman Bold