I have a document library with three allowed content types - A, B, and C. A inherits Document Set, B and C inherit Document. Using A's Document Set Settings, B and C are the allowed content types within A. A does not share fields with B and C. I want to create a custom edit form (and newdocset but I'll post that separately) for A.
Using Designer, I created a new Edit Form (EditFrom_A.aspx) for type A and did not check "set as default" since I have other content types in the library. I saved the full URL of this form in MyLibrary > Content Types > A > Forms > Edit Form. Now when I click Edit Properties for a particular instance of A, I get a blank modal popup. However, if I copy/paste the URL into the browser and add the ID as a param, the page loads successfully. Instances of B and C still load the OOTB Edit Form correctly.
Then I tried setting EditFrom_A as the default Edit Form for the library. After removing the URL from MyLibrary > Content Types > A > Forms > Edit Form, it works fine for A. Of course, the same form is now used for B and C. I tried setting their Edit Form URL to the OOTB form but the modal popup is blank when I click Edit Properties.
Any suggestions? I'm going crazy trying to figure this out...