If you see a submission that has failed in the portal with an error similar to this:
Execution Failure: Error connecting to remote server on Forms-Based authentication: Connection timed out (Connection timed out) the form has taken too long to send to the server and the connection has timed out.
In most cases this is just a connection issue at the time of the submission. The administrator on the account should be able to click the drop down arrow beside the form submission ID and click "Re-process Upload". The submission should successfully go through after this.