Why is openDoc working in Reader XI but not Reader DC?

I am going nuts trying to figure this out. I've browsed past forum posts and none of them seem to apply to my particular situation.

I have a form ("primary.pdf") I created in Acrobat XI with a button that runs a javascript on Mouse Up that takes its form data and inserts it into another form I created ("secondary.pdf"). The two files are in the same directory on a Windows network server. The javascript on the button has the following line 38:

app.openDoc("secondary.pdf", this);

When primary.pdf is open on a windows client running Reader XI, I can click the button and the script works great. When primary.pdf is open on a different windows client running Reader DC, I get the following error output in the console:

NotAllowedError: Security settings prevent access to this property or method.

App.openDoc:38:Field Button1:Mouse Up

The only way it will work in Reader DC is if I disable Protected Mode at startup (in Preferences -> Security (Enhanced)), but this has obvious negative ramifications.

I also noticed that if if secondary.pdf is already open in another Reader DC window/tab, it will not throw an error; if I click the button in primary.pdf with secondary.pdf already open, then it runs just fine.

Additional info:

• The 'disclosed' property on secondary.pdf is set to true (document level javascript), but it still does not work
• Both files are signed with my certificate and have been set as Trusted on the client machines running Reader XI and Reader DC. Even if I add the files to the Privileged Locations list for Reader DC, I get the same error.

Any ideas??