Hello @ninonbane,
The PHP Warning message indicates that an array (the processing of a series of data) is empty. That specific line of code relates to the sign up form fields, meaning that it detected no data coming through. At first I would suspect that there were no form fields (name, email address, etc) configured for the sign up form in the Widget configuration. However, since you say “when I fill in the form” it sounds as though you do have form fields showing up to fill out. That makes me suspect another plugin or the theme itself may be blocking the form fields.
This is not a bug we have seen before, so I’m curious what the cause is. The best way to isolate it is to temporarily disable ALL plugins and switch to another theme to see if the problem reproduces. Your web host may offer a Staging environment where you can do that without disturbing traffic. Otherwise you could use a development site running locally, or another copy of your site running somewhere, or just do it live. Once the problem is not reproducing, enable your theme and plugins one at a time while re-testing each iteration to see which one is the trigger. Once I know what triggers is, I can attempt to reproduce the problem and see what might fix it.
We can also get a temporary account to look at your site directly, but with all that can go wrong on an unfamiliar site that comes with risk and we claim no liability or warranty with doing that. Let’s try having you work through this a bit before we resort to that.
Thanks for using our plugin, and for reporting the trouble to us so we can help you resolve it shortly.