Hi there,
I am very sorry for the inconvenience
Unfortunately, this is a known bug. Our engineers are aware of an issue where the email field throws an error. If the email field is blank, it validates as an invalid email address upon submission, even if the email field is not required. I have notified them of your case to keep it on their radar.
In the meantime, you can download Ninja Forms version 3.8.15 from https://downloads.www.ads-software.com/plugin/ninja-forms.3.8.15.zip and install it manually via the Plugins > Add New Plugin page using WordPress’ Upload Plugin functionality until a fix for this has been released. I do not have an estimate regarding when a fix will be ready. I sincerely apologize.
Please let me know if you have any questions or if there is anything else we can help you with.
Thank you for your patience.
Have a good day!
Thanks
Faisal