I can't reproduce this. Here's what I did:

• Found a project.
• Clicked "Subprojects".
• Clicked "Create Milestone".
• Filled in the details, clicked submit button.

This created a milestone:

Do you have any idea about what I did differently?

Yeah i did basically the same. What are the permissions of your project?

Generally easier if you just tell us what your settings are that reproduce the issue, rather than us try to reverse engineer all the choices.

I've posted a screenshot of my settings or do you need anything else?

I don't know the relation of your account to those policies. Are you a member of those groups?

Yes I'm member of all those groups as described above. I'm also an "Admin"

No luck, I tried the same settings you just posted, an admin as edit policy, also in the group view and join policy. I couldn't reproduce your screenshots.

Ok, then I'll investigate further and give feedback as soon as i've found something

Ok i found the reason for the problem.
If you set Applications → Projects → Edit Policies → Default View Policy to Project Memebers i does not work. You can still create normal projects and subprojects but you can't create milestones. I think that is because milestones do not have members but the permissions are evaluated against the "project members".

So PhabricatorApplicationTransactionEditor::validatePolicyTransaction fails...

Can you reproduce and confirm that?

I'm going to guess this is the same as T10779

I know this was linked against my question, but I wanted to check and see if it is the same thing...

@sascha-egerer, did the project in question have at least one Subproject (differentiating from Milestone, of course...they're two different things.)

@CodeMouse92 yes the project has a subproject. But changing the policies as described above works around that issue.

@sascha-egerer that would seem to be the main factor, and the reason this couldn't be duplicated initially. It isn't all milestones - it is specifically milestones for projects that have subprojects.

You had half the problem figured out. Adding a subproject migrates project membership to that subproject, and then the main project's membership is a combination of all of the subproject memberships.

PhabricatorApplicationTransactionEditor::validatePolicyTransaction fails if...

1. The parent's members list is based solely on its children's members lists. (It has Subprojects.)

2. Permissions are set to Project Membership.

So, in short, yes, this is indeed the same thing as with my bug.