I’m having the same issue
I looked in the FAQ and it said:
“When a user receives “Invalid Nonce” after manually registering and filling out all the details on your site, it is basically caused by the expiration of the nonce. Please note that this is part of the security feature of the Ultimate Member core plugin.
To resolve this issue, simply exclude registration pages from the cache, especially when you are using WordFence.”
https://docs.ultimatemember.com/article/230-invalid-nonce-in-registration-form
I don’t know how to exclude pages from the cache
I even installed WordFence and added it to the whitelisted IPs but it still gives me that.