In one existing site collection all document libraries are suddenly showing the _cts folder.
Any idea how to fix it?
For others searching for an answer to this problem, we eventually fixed the issue like this:
In the list definition where you define the content types, in the targetName attribute we were specifying the content type name like this: "_cts/MyContentType"
We've changed it to exclude the "_cts" part and now the issue no longer shows up (and functionality still works as expected)
EDIT: This fixes the issue on newly created sites, to remove it on existing sites you need to remove them manually (or use a PowerShell script).
-cts folder belongs to so-called hidden URL structure of a sharepoint web site.
By default only users that (or who) are members of the Site Owners group and Designers group see it (as well as All Files tab in Sharepoint Designer 2010).
In Sharepoint Foundation the Designers group is not created by default at all.
Site collection administrators also can be limited but at the site collection but web app level.
At site collection level
these settings are configured, in browser, at top level site of site collection (navigate to root site of site collection) > Site Actions > Site Settings > Allow Site Owners and Designers to See the Hidden URL structure of their Web Site .
At web app level
it is configured in Windows Start > All Programs > Microsoft Sharepoint 2010 Products > Sharepoint 2010 Central Administration > Manage Web Applications under Application Management section > choose a specific web application > on ribbon, the Web Applications tab (under Manager group section)
I thought that _cts virtual directory is for site only, maybe you activate new content type or document set via feature for whole site. Is it accessible for users? Is it empty?
Try using Request Filtering in IIS, click the hidden segments tab, and add _cts.