The cURL error can impact add-on feed processing, which occurs in the background, but it doesn’t usually impact form submission completion. The following article includes advice for resolving it: cURL error 28 in WordPress – Samuel Aguilera
The most common cause of the submission stalling with the spinner active is a theme or plugin conflict.
Have you tried running through a complete conflict test using the steps outlined on this documentation page: Testing for a Theme/Plugin Conflict?