A 400 error will typically occur if the client (i.e. your browser) makes a bad request to the server. This could be a malformed request or sometimes a request that the server cannot or will not handle. 400 errors are not caused by WS Form.
Here are some common causes and solutions for a 400 error.
Cookies & Caching
If your web browser cookies and/or cache become corrupt or invalid it could cause this error.
A simple way of checking for this is to open your WordPress website in incognito mode and see if that remedies the problem. If it does, we recommend clearing your browsers cookies and cache.
It is also possible that caching plugins and/or server caching could be causing this issue, so try temporarily disabling any caching in place to see if this remedies the situation.
Large Uploads
Servers are typically configured with upper limits on the size of requests that can be uploaded to it. Having a file upload field on your form can greatly increase the size of the request when the form is submitted. If this exceeds the configured limits, a 400 error will occur. Check with your hosting provider as to the max limits set in place.
Anti-Virus Software
Client side software such anti-virus software (e.g. BitDefender Antivirus) that manipulates requests from your web browser before they reach the server can cause 400 errors too. Double check your settings in that software to make sure it is not causing malformed requests to occur.