> I uploaded a beta that might fix it, but it needs more testing. The
> issue might have been introduced when support was added for more complex
> ligature building, compare for instance this:

Thanks Hans! Indeed, it was a fix, but there are still problems I find 
in further testing, see attachment for this code:

\definefont[amiri][file:UthmanTN1Ver10.ttf*arabic at 17 pt]
\define[1]\Arab{\setupalign[r2l]\amiri#1}
\starttext
\Arab{
لرَّحْمَـٰنِ
}
\stoptext

In context the two signs are set one upon the other ("sukun" and 
"fatha"), see the second attachment (word) for the correct setting. But 
now this has nothing todo with ligatures, deactivating them does not 
help now.

Huseyin