jlapoutre

Damn! I guess you must be right; just before upgrading Polylang I tried to upgrade WordPress to 4.0.1, which didn’t work.

Sorry, this might not be a bug at all.
Investigating more shortly…

Exactly the same here for

WP v3.5.1
polylang v1.0.2

Theme: Twentyeleven (tested with theme version 1.3 and after upgrade to current v1.5).

Only affects randomly rotated uploaded header image, not one fixed uploaded header image.

For now I use just one header image, site: https://www.leidsamateurkunstfestival.nl/en/