I can't reproduce this.

Strange. In one project I have 3 milestones and I can tag only one. In other project I have 2, but I can't tag any of them.

Is there any script which check if the database is not corrupted?

You're welcome to use whatever database tools come with whatever database you're using to check for corruption, but not likely the issue.

If you feel there is a bug in Phabricator, please spin up an instance on http://phacility.com and verify the bug with complete steps we can follow to reproduce the issue.

I will and let you know. Thanks for helping me!