500 Internal server error when trying to remove invited user from project's members

I already deleted the “invited user” from the admin dashboard, but he still exists in the project’s members list. When i tried to remove it from there as well, i got 500 internal server error!. I tried to remove it from the gitlab shell, also failed!

GitLab information
Version: 13.7.1
DB Adapter: PostgreSQL
DB Version: 12.4

GitLab Shell
Version: 13.14.0

Any help please?