I'm pretty sure this was either an upstream bug or we fixed this at one point and it probably broke somewhere between a version or moving to HHVM+Nginx. Possibly even the distro upgrade...
I have seen the exactly same issue earlier (outside ShoutWiki), although way before 1.27.0 (iirc we were also running HHVM back then).
The solution was to install ttf-freefont (should be available in Debian and Ubuntu), and putting this in LocalSettings.php:
putenv( "GDFONTPATH=/usr/share/fonts/truetype/freefont" ); $wgTimelineSettings->ploticusCommand = "/usr/bin/ploticus"; $wgTimelineSettings->perlCommand = "/usr/bin/perl"; $wgTimelineSettings->fontFile = 'FreeSans';
Please try this and let me know if that fixed it.