I can understand your concern. Allow me to explain a bit, Previously whenever this issue occurred we always tried to get details about replicating the issue. All the previous users somehow were not able to provide the exact details, hence it was difficult for us to identify the problem. Since we do not have any access to the site (usually we avoid asking for credentials), we ask users to deactivate the other plugins and let us know the results. Since now we have a form where this issue is replicated by following certain steps (Thanks to blandersfsu), We should be able to resolve it soon.
Many thanks for looking into this! Unfortunately, I do not have good news for you though. I went ahead and deactivated all plugins on my site and still encountered the error. I even switched to the stock 2020 WordPress theme with all other plugins disabled and the error persisted. So, unfortunately, I can confirm that it is not a plugin or theme conflict that is causing the error.
In our further analysis of the problem we found that a cache plugin can cause this issue. Every form generates a security token while rendering the fields. Then this token is validated at the server side whenever submission happens. It allows to prevent bot submissions. However we noticed that some of the cache plugin affects this behaviour. Cache plugins renders the form from cached copy.That causes the submission to be rejected by the server as token is not generated dynamically.
Solution is to exclude the form page from being caches.