No activation in Forms (2.4.3.3) possible

Hello,
I have performed a fresh installation of Joomla 6 (6.1.3) on my web space and also installed "Gridbox" (version 2.20.3.1) and "Forms" (version 2.4.3.3).
I have already activated my Gridbox license. Now I am trying to activate the license for "Forms." A pop-up window for the username and password appears, but as soon as I click inside the window—for example, to enter the username—the pop-up box closes immediately.
At the same time, another pop-up window displaying the message "Error" appears in the bottom right corner of the screen.
What can I do to activate my license for "Forms"?

Thanks for your help.

Best regards,
Frank