Attaching two or more files to a project Wiki gives a 503 error

Problem: After upgrade an Omnibus Gitlab 12.5.4 to 12.6.4, If I try to attach several documents on the wiki of any project I obtain a 503 error. This is an on-premise installation over CentOS 6.

What are you seeing, and how does it differ from what you expect to see?
I can select several files and it seems the upload process starts but after some seconds I can see a 503 error.
After reloading the page I can access de wiki but not files were attached. If I do the same process just using one file there is no problem.

  • What version are you on (Hint: /help) ? and are you using self-managed or
    Version 12.6.4

  • *What troubleshooting steps have you already taken? **
    Uploading one file a time there is no problem.