I have a self-managed GitLab CE v13.1.3 on CentOS7 (omnibus installation).
With the last update (the previous version was 13.0.4) “New Project” button disappeared from the Groups pages (all).
Only from Admin Area, “New Project” button is available, but selecting any group (also a new group) as namespace, the New Project Creation fails with the error “The form contains the following error: Namespace is not valid”.
I tried also to transfer a project to another group, but it failed too.
From GitLab logs no useful information I fetched 
Any ideas or suggestions?
I found the misconfiguration.
The issue was related to " Default project creation protection " setted to “No one” (see #217966)
Note that this behavior is changed in last GitLab release : I never changed it from the first setup of my GitLab configuration as “No one” and only in the last release I’m not able anymore to create projects getting the wrong error message “Namespace not valid” .
Furthermore, also project transfer is affected to this configuration and goes in error.
At this moment, “No one” includes also Administrators, but the meaning/behavior of this configuration doesn’t make sense and it is unuseful.
I suggest to change the meaning/behavior substituting “No one” with " Only Administrators " and reviewing the Project Creation and Transfer features and also the error messages that should report the current configuration setting.
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@beninia Thanks for coming back to the forum to share your solution. And, for commenting on the issue! That is the best way for our product team to see this feedback. Thanks for your contributions!