OS X Syriac script/font not rendered
Not a perfect solution but now I found a way to have the characters rendered in all programs.

From the comments to the original question I assume that an installed font is poisoned by wrong glyphs or defect in some other way. For a quick diagnosis I installed Ultra Character Map. It didn't help much to locate the defect font, but used the built in Google Noto Font to render characters.

Before diving deep into writing Perl scripts for font analysis, I installed all Google Noto Fonts into user fonts and everything works now, also Dhivehi.