Posts on frontpage are text only

Hello

I update an intranet site, hence no link, to wordpress 5.6 and got the black screen error. Updating the theme resolved this issue but the recent posts on front page are not loading correctly. It used to show full posts with images and text formatting, now i get only simple text. Posts are displayed correctly individually.

I tried disabling all plugins and the issue remains. With other themes like TwentyFourteen or TwentyFithteen the posts are displayed correctly on the frontpage.

Thanks in advance!