First of all, i want to thank @Tanguy Crusson and the Atlassian community for their willing to help.
My issue
After assigning an idea to an assignee, it is resetted the whole time with the following message:

The message is confusing, because the idea is already created. Just want to update the assignee field. But everytime i reload the page, the assignment got lost. What do i do wrong?
Help is always appreciated 