XF 1.5 mixed up user-selected language in frontend

[high1976]

we have an issue lately with posts and user-selected languages. Very rarely post-links (edit/delete/report/quote) have a different language then users default (or forum-default). This doesn't affect the whole page, just one post per page (at least we havn't spotted more so far). Screen:
users language in this case is set to german while 2nd post shows english links while time is still german. Happens the other way too with all languages installed (5). A page-reload solves the issue every time.

Tried several caching settings (XF/php/nginx) including disableing everything that smells like caching but no luck so far, users are still reporting it. Any idea whats going wrong here or what might affect/cause that issue?

 

[Paul B]

The first step would be to disable all add-ons and test in a completely default unedited style.

It's not something I've heard of before.

 

[high1976]

tested already for some time but we where never able to reproduce it on an 1:1 identical testsystem, neither with default/no plugins or with the full set. It happens *very* rarely and it seems not to happen (at least no one spotted it) in a environment where just a few couple of users are active. Live-system has a few hundred users online at once but enough free resources so not caused by some kind of overload.
Initially thought it's cache-related, maybe it's somehow cached from a user viewing that specific post with a different language a moment ago but no luck disabling everything or trying to reproduce it with several users with diff. languages.

 

[high1976]

12 months later and users still report it ;( Maybe anyone else noticed it in between and has some new thoughts/hints about it ?