There could be two possible reasons for this to happen: You have not set the secret key on WordPress and Moodle software: If this is the case then refer to the documentation to learn how to set the secret keys on both websites.2. You do not have the mcrypt extension activated on both WordPress and Moodle software: Install and enable mcrypt extension on both websites to resolve this issue.3. The issue could be due to file permissions related to the directory and files in SSO. (Ideally, the “Directory” access permission should be set at 755 and the “Files” access permission should be set at 644)