WHMCS Module Order Accept Encountered Problems

Hello,

No matter what and how I do, I still get that error.

I've whitelisted the IP, where WHMCS is installed.
The API Manager also has the correct values.
Everthing else is well set up. But it just don't want to work.

FYI, I am using the latest CWP7 module..

My version of WHMCS is 8.1.0

Please advise.