Package: x2goserver
Version: 4.0.1.19-0x2go2+git20160126.1064+stretch.main.1

x2goserver depends on x2goagent which depends on nxagent which depends
on libxcomp3 which depends on libpng12-0 which is flagged obsolete.

Please update packages dependency (and maybe adjust source) to depends
only on libpng16-16 (version 1.6)

Thanks,
Best Regards,
-- 
Thierry MURGUE.