Hi @dix93,
We appreciate your findings.
The WP User Frontend plugin is not 100% compatible with the multisite installation/network active mode. In this case, you need to just activate the plugin individually instead of the plugin network activation. Also, allow registering a new user from the Network Settings on your site as you can see in the screenshot- https://d.pr/i/Kgp8o7.
We’ll look into this and I logged it as a potential bug on our GitHub repository if you would like to follow the issue.
Best regards.