Step by step approach to finding Javascript Conflicts
1. Switch temporarily to a WordPress default theme (Twenty Eleven etc.) and see if the problem goes away.
If it does, you’ve most likely got a JavaScript conflict with your current WordPress theme.
If switching to default theme doesn’t help it could be one of the plugins you are using that is causing the problem.
2. Disable all plugins other than Contact Form 7
If doing that solves the problem, you can then add back each item – one by one, until you find the real cause of the problem.
?????RE: Javascript might be likely not to blame as the contact form arrow spins during sending.
??On the contary – this is a common indicator of a Javascript Conflict.
RE: if its an issue with the theme, why do the error messages appear correctly and not the send confirmation?
??
??The CF7 Javascript that displays validation error messages runs early in the email sending process. Your particular Javascript Conflict probably occurs after this point.
RE: Never was a problem prior WordPress 4.0 as much as I recall.
????Possible that your theme or one of your plugins is not compatible with WP 4.0
You will need to disable, temporarily, ALL Javascript, other than that from CF7, including anything in your theme (switch temporarily to a default theme), if you actually want to get to the source of the problem.