Creating an issue in gitlab-ce results in a 405 Method not allowed

Hi all,

I’ve managed to migrate gitlab 8.1 to 8.2, but I get an annoying issue.
Everytime I try to create an issue, I get this error :

==> /var/log/gitlab/gitlab-rails/production.log <==
Started POST "/api/api/issues" for at 2015-12-15 15:05:13 +0100

==> /var/log/gitlab/nginx/gitlab_access.log <== - - [15/Dec/2015:15:05:13 +0100] "POST /api/api/issues HTTP/1.1" 405 2 "" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.80 Safari/537.36"

It only occurs on this project. Do you have any idea where it comes from ?
For information, I use a custom nginx


Edit: To give more details, I have a white page like this when creating the issue :