Thanks for the detailed answer and the WP Discourse template customization link, I used some code from here and now featured images work again, among other things that were also broken. Everything was working well less than a week ago so I guess my code was incompatible with a recent update of WP Discourse or something like that.
However, there’s still the featured link error in WP Discourse logs though posts are correctly updated. 
Despite this logged error, everything seems to be working fine.