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The answer from a SE question has provided a little light:
http://tex.stackexchange.com/q/274582/2148
Directories listed in OSFONTDIR are not recursively scanned for font files.
That answer applies to kpse which Context isn’t using for path scanning. In fact, with the beta 2015.11.19 19:13 I get: $ OSFONTDIR=~/.fonts mtxrun --script fonts --reload … fonts | names | identifying system font files with suffix 'otf' fonts | names | 'OSFONTDIR' specifies path '/home/phg/.fonts' fonts | names | globbing path '/home/phg/.fonts/**.otf' fonts | names | identifying system font files with suffix 'OTF' fonts | names | globbing path '/home/phg/.fonts/**.OTF' fonts | names | 468 system files identified, 155 skipped, 155 duplicates, 313 hash entries added, runtime 3.186 seconds … The fonts are indexed accordingly: $ mtxrun --script fonts --list --file SourceSansPro-Regular.otf resolvers | trees | analyzing 'home:texmf' sourcesanspro normal normal normal normal sourcesansproregular /home/phg/.fonts/OTF/adobe-source/SourceSansPro-Regular.otf conflict: regular And Context will resolve those fonts, initially. However, the font can’t be found at definition time and later for embedding: $ context fnt.tex … fonts > otf loading > loading '/home/phg/.fonts/OTF/adobe-source/SourceSansPro-Regular.otf', hash 'sourcesanspro-regular' … fonts > otf loading > loading done fonts > otf loading > saving '/home/phg/.fonts/OTF/adobe-source/SourceSansPro-Regular.otf' in cache system > lua > compiling '/home/phg/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/fonts/otl/sourcesanspro-regular.tma' into '/home/phg/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/fonts/otl/sourcesanspro-regular.tmc' system > lua > dumping '/home/phg/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/fonts/otl/sourcesanspro-regular.tma' into '/home/phg/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/fonts/otl/sourcesanspro-regular.tmc' stripped fonts > otf loading > loading, optimizing, packing and caching time 0.569 fonts > defining > source file 'SourceSansPro-Regular.otf' is not found … close source > 2 > 3 > /tmp/ctx/fnt.tex close source > 1 > 3 > /home/phg/context/tex/texmf-context/tex/context/base/cont-yes.mkiv !LuaTeX error: cannot find OpenType font file for reading () ==> Fatal error occurred, no output PDF file produced! Log attached. Bug? Best, Philipp