Hi @alex.hoebart. When you get a chance, please introduce yourself here so we can learn more about you and your project!
I've seen this kind of problem when folks run into Apache or Aggregate or MySQL/PostgreSQL timeouts. This could be because you are on a slow (or high latency) connection or have large attachments or some combination of that. I'd start by looking at your logs and see if there are any hints of timeouts or post limits.
It'd be helpful to also know the following:
- Are your users trying to use HTTPS when the server (or submission_url in the form) is configured for HTTP (or vice versa)?
- Are your users in a network environment where there might be proxies or government firewalls?
- When users get the 400, what fixes it (e.g., reboot, change from WiFi to cell network)
- What version of ODK Collect are you using?
- Does it ever happen with Enketo?