Error message "not allowed"

Hello!
I have installed the Ultimate Member plugin. I use a seperate page as my registration form, which I have added in the setup area of the plugin. When someone tries to register, after he passes the field checks (alphanumeric, min-max size, unique u/n, difficult password) it is reloading the same page but insted of showing him a welcome message, he gets “This is not allowed for security reasons.” and the user is not registered. The username is not admin, it is anything normal.

Do you have an idea, how it can be fixed? Thanks in advance.

https://www.ads-software.com/plugins/ultimate-member/