Error page during a translation

Hello, I have been using the Polylang plugin for some time. Unfortunately I discovered an error today. When I activate French I get an error page. strangely, only in French. The English translation works without any problems. I’ve already updated the permalinks and cleared the cache. All without success. Does somebody has any idea?

https://www.schloss-sanssouci.info/

Translation: https://www.schloss-sanssouci.info/fr/

thx