I've already submitted this as a bug on Gemini, but wanted to share this with others.

In DNN 5.1.0, if you attempt to Add Existing Module for a module that had been on a page and is now in the Recycle Bin will fail with no indication of the problem.

If you remove the module from the Recycle Bin, you can Add Existing Module as normal.

Steps To Reproduce:

• On any page in your site, use Add Existing Module to add a module to that page.
• Delete the module you just added
• Attempt to use Add Existing Module to add that module again.

In DNN 4, it worked as expected.  It looks like the issue is that in DNN 5.1.0, when you delete a module reference, it is put in the Recycle Bin (which didn't happen in DNN4...it simply disappeared).