Was the form submitting OK in the past and you’ve created a new version and added in new features/functions. if so, review the changes you have made.
Have you some specific ‘scripts’ on the form that run in certain situations where you see this but other times you can submit OK
also look to run the form in ‘debug’ mode to see if you are able to see some form scripts that have a problem
Failing that, please send over an export of your form in the support ticket with explanation of any specific ‘actions’ required to replicate the issue and we can have a look