re: error-log; depends on your hoster's configuration, really, but if the file you looked into has PHP-errors, -warnings and -notices, then yeah, that one.
the cache info makes sense; your main site has cached styles & scripts because AO works, the other ones don't because somehow AO does not get triggered. I can create a autoptimize.php with some debug code in it, if you're interested? That way we would at least have the start of an idea of why AO isn't autoptimizing your subsites?
frank