Upload odk build form - error message doesn't make sense

What is the problem? Please be detailed.
I receive the following message even when I use an absolutely simple form, i.e., a numeric and a text block, to upload to AGGREGATE: Could not upload the form. Aggregate could not validate the form contents. Please make sure your form is valid and try again.

What ODK tool and version are you using? And on what device and operating system version?

What steps can we take to reproduce the problem?

What you have you tried to fix the problem?

Anything else we should know or have? If you have a test form or screenshots or logs, attach here.