Error message on return email

I have Front End Pm and love it on my site but I needed something that could only be used for Admins and clients. I do not want the clients to be able to message each other. I was thinking the new FEP Contact Form was going to be just what I needed but I am getting an error message.

I set up an admin account and a test client account. I was able to message the admin from the client account but the admin cannot message back. I get the following message.

Error: Sorry, your nonce did not verify!

For now this site is a demo for a client and I have about 72 hours before he will see it. It then has to be remade on his new site, if he approves the design. So it should take about a week but if you could respond before then so I can test it on the demo that would be great if not I will dig through the code myself. I know this is a side job for you but it is a great concept and the FTP works very well on my other site I just need this one to work too. 🙂

Thank you for your time and your wonderful plugins.

https://ww.wp.xz.cn/plugins/fep-contact-form/