Webform fails on preview with 403 error in console.

I have a multi-page webform using version 7.x-4.12. I have a few users reporting that when they complete the form and press the preview button, that they are re-directed to the start of the form and all their input is deleted. The issue occurred before and after my update to 7.x-4.12. I did a normal module upgrade through the UI. The issue is still happening, and one of the users has tried with multiple browsers.

I'm not sure where to go from here to try to diagnose or fix this. Any advice is appreciated.

UPDATE: I checked the error logs on the server and didn't get anything meaningful from there and no errors recently.

In looking at the logs, I can see sessions getting opened and closed for both of the users.