Hi @alptkz,
Thank you for your reply.
I could reproduce the issue on my test site using latest version of Membership plugin therefore i have notified the plugin developer to fix this issue.
In the meanwhile to fix this issue try editing the following Membership plugin file as below on line number 134
Membership plugin file:
membership\membershipincludes\addons\simple.invitecodes.php
Before Editing:
if(empty($error)) {
After Editing:
if(is_wp_error($error)) {
Note: You are making changes in the plugin file and these changes will be overwritten and lost when you update the plugin and you will have to make these again after plugin updation.
Best Regards,
WPMU DEV