I was having a similar problem with the arrow spinning and error handling not working. I hope that my solution can help someone else. I was ripping my hair out because it had been working perfectly a few days before. I had been fine tuning the site in prep for a release, but still had the problem after disabling all newly installed plugins.
If I removed the file upload fields from the form it worked - but I needed those!
Finally I narrowed it down to a conflict with an embedded MailChimp form I had in a sidebar widget! I had used the embed code because none of the MailChimp plugins I had tried did what I want. I didn't even consider the form when troubleshooting because it wasn't a plugin.
I used Chrome's tools to watch the console errors - that led me to a javascript error in a MailChimp file. Problem solved!