Strange 502 error and language

Hi there,
I’ve recently set up a news magazine WordPress and everything worked fine until some of the authors started complaining about random 502 errors. Although the majority of then were random there was one that was persistent in some: Appearance->Customize. I tried everything from siteground’s and cloudflare’s support but everything seemed ok. Until the owner of the website (!) thought that it had to do something with the fact that the site’s language is greek and his interface’s language was english. Changed it to greek and boom! All ok! Has anyone faced anything like this before? Is it a bug? Is it a siteground’s security measure that results in 502 when the two languages don’t match?
I really have to wrap my head around this…
Thanks in advance