Sorry for the delay, had a broken wrist recently and reduced screentime significantly.
After checking today, I have to report more unusual behaviour. Besides the fact that the setting seems to be correct on the main page https://einhorn.bar/:
<meta property="twitter:url" content="https://einhorn.bar/">
it seems that other pages have other settings, for example
<meta property="twitter:url" content="https://mail.zum-tanzenden-einhorn.de/reservierung/">
on the page https://einhorn.bar/reservierung/.
The subpage https://einhorn.bar/events/pubquiz/ shows
<meta property="twitter:url" content="https://altpeter.de/events/pubquiz/">
as property.
It seems like the plugin is using random FQDNs from the server. I don’t see a pattern, though.
The following plugins are currently installed and active:
ActivityPub
Colibri Page Builder PRO
Custom Contact Forms
Disable Comments
Easy Updates Manager
Flexi Pages Widget
iframe
Insert PHP Code Snippet
Loco Translate
NextGEN Gallery
OG
PDF Embedder
Reusable Blocks Admin Menu Option
Site Kit von Google
Social Media Widget
TablePress
Theme Switcha
Ubernyms
Use Any Font
WP fail2ban
WP Hydra
WP Links Page
WP Mail SMTP
WP Render Blogroll Links
WP-Paginate
WP-WebAuthn
XML Sitemap Generator for Google