All post excerpts are suddenly the same

Very strange issue.
My customer created a new blog post and all of a sudden, all previous posts show the same excepert on the standard archive page (https://ein-neuer-weg.com/blog )
This happens with all plugins disabled and even with a standard WP theme.
It does not affect archive pages I create with BeaverBuilder, using their "Posts" module. The single posts are not affected.
The excerpts shown are always from the latest post, so if I delete the newest post, all excerpts change to the next one.

Any suggestions how to sort this out?

WP 5.0.3, BeaverBuilder (Plugin, Theme and Themer), Polylang.