Currently able to reproduce this in mwrepl with:

break Scribunto_LuaSandboxInterpreter::callFunction()
$ctx = new RequestContext();$ctx->setTitle(Title::newFromText('Thomas_Mann')); $ctx->setRequest(new FauxRequest(['title'=>'Thomas_Mann','diff'=>'10758157','oldid'=>'9788822']));$article = new Article(Title::newFromText('Thomas Mann')); $article->setContext($ctx); Action::factory('view', $article, $ctx)->show();
c 35

That will, currently, stop in the function that then calls into luasandbox and errors out. Probably a very fragile way but couldn't find anything better.

It doesn't have debug symbols for /usr/lib/x86_64-linux-gnu/liblua5.1-c++.so.0 Those are provided by the liblua5.1-0-dbg package.

FWIW, I managed to work around the issue by optimizing some modules so that they load less Wikidata items. However, the error should be better reported.

This is the change that I made in order to avoid it. That module was loading up the Wikidata item of the city of birth in order to get the country where it was located. If you want to test it, just rollback my change and you can test live on ro.wp. We're a small community and we discussed the issue at the village pump, so just mention you're investigating it when you rollback my change and the patrollers should leave you alone. Thomas Mann is the only article where we encountered this so far, anyway.

What I have been able to determine/guess here:

• The only place I see "unknown exception" as the only text of an error is from runtime/vm/unwind-inl.h in HHVM. It doesn't appear in MediaWiki core, Scribunto, other deployed extensions, luasandbox, or lua-5.1.
• Based on what little bit of stack info we have here, my best guess is that #5 is this line.
• I see some other instances of the same error in the logs (kibana link), but so far not one that's reproducible. No idea if they're the same cause or just the same error message.