Error message that failed to send but E-mail sent successfully

Hi,

I have an interesting issue.

If I try to send form, I got a red bordered error message, that “Failed to send your message.” but after that I got the e-mail successfully.

Please help me, how can I solve this issue?

I didn’t see errors in server logs’, and the browser’s Console is empty, too.

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