Malfunction in Yoast's WordPress SEO

Entering ‘Check’ on Yoast’s ‘WordPress SEO’ plug in should bring up a menu including ‘Page Analysis’. This worked initially, but then stopped working for no apparent reason on my English website; instead of a new page including the menu, I get text at the bottom of the page as follows:

URL/wp-admin/post.php?post=31&action=edit#wpseo_linkdex

I have a second website in German that is identical (including plugins) except for the hosting site and the language. Deactivating other plugins doesn’t help.

I would be grateful for any suggestions.