Spinning wheel keeps spinning after submitting the form

After submitting the form the spinning wheel keeps spinning, the mail is not send, but it does appear in the CFDB7 list.

This happens after updatint to Verion 5.8.4, before that it worked well.

After downgrading from php8.0 to php7.4 the problem was gone..

The page I need help with: [log in to see the link]