No page access since 10 days to: https://commons.wikimedia.org/wiki/Category:SVG_logos_of_the_United_Kingdom !?
But **only if logged in** (probably due the default English language, see below), testet with Firefox (give 503) and Chrome (give 500) on two different machines (Win7).
Request from xx.xx.xxx.xx via cp3042 cp3042, Varnish XID 867253101
Error: 503, Service Unavailable at Sat, 22 Jul 2017 21:38:12 GMT
So this is probably a gadget error or similar user setting.
Edit: Some people reported a relation with the language setting:
So this server error only appears on **non English** language setting.
**Workaround ** is to switch to EN (clear cache and switch back to the user language).