Thank you verry much for your efforts!
I have closed all tabs and restarted Opera from scratch. Bitdefender still gives the following message, but I have translated it from German here.
opera.exe has attempted to connect to ftpcloud.de based on a mismatched certificate. We have blocked the connection to protect your data because the certificate used was issued for a different web address than the destination address.
Translated with www.DeepL.com/Translator (free version)