Have you tried using encodeURIComponent on the form data before sending it? Most clients do this automatically, but maybe this plugin isn’t.
As you may know, the ?, & and = signs are used to delimit form fields and may be truncating the data parsed by the server.
The other solution is to fix the root CORS issues on the server. Check out the new CORS Errors page in the docs to find an appropriate solution for your use case.
Best,
Rodrigo