Did some investigation on this:

• The error is emitted not by WMF sites but by some intermediate DoCoMo proxy, we don't output 502 errors in Japanese.
• It happens only upon redirecting from desktop site to mobile.
• Early November coincides nicely with our switch of desktop sites from Squid to Varnish - this could have slightly altered responses.
• I couldn't reproduce this bug in their simulator: https://www.nttdocomo.co.jp/english/service/developer/make/content/browser/html/tool2/download/

I'm not sure if this is relevant, but some (who experience the same problem) suggest emitting the "Content-Length" header in the response to deal with this 502 error: http://umakoya.com/blog/log/eid179.html

According to the specifications given by Docomo, their browsers (and proxies, too?) require the "Content-Length" header: https://www.nttdocomo.co.jp/english/service/developer/make/content/browser/xhtml/notice/basis/

bingle-admin wrote:

Prioritization and scheduling of this bug is tracked on Mingle card https://wikimedia.mingle.thoughtworks.com/projects/mobile/cards/1750

Change 117471 had a related patch set uploaded by MaxSem:
Always output Content-Length on mobile redirect

https://gerrit.wikimedia.org/r/117471

Change 117471 merged by Faidon Liambotis:
Always output Content-Length on mobile redirect

https://gerrit.wikimedia.org/r/117471

Let's see if this patch fixes the problem. Please reopen if it doesn't!