Thank you for reaching out to Constant Contact's API Support.
It is possible that the timeout setting is just not long enough to allow for the form to be submitted. Especially if resubmitting the form is working correctly. I would be interesting in seeing the form and/or backend code to see if there might be something else going on. If you do not wish to share that information publicly you can email us at firstname.lastname@example.org.
Regards, Jimmy D. Tier II API Support Engineer
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