Spinner after submit form

What problems do you find with the form?

The spinner icon shows up once I submit a form, but then nothing happens (form is submitted to the database but no e-mails sent and no success message). This form for example works fine: https://becker-immobilien.com/mieterselbstauskunft/. And I can′t see no difference in the settings. When I create a new form, I have the same issues.

FAQ > The spinner icon shows up once I submit a form, but then nothing happens.

Having the same problem. Spinner keeps spinning after submission. Sometimes the form is submitted, and sometimes it times out and is marked as spam.
Disabled all plugins except c7 and switch themes, and the problem persists. The issue seems to be recent; perhaps after update to 5.8.
Any solution to this?