Adobe Reader gets stuck with "Initializing" message when used through COM / ActiveX

In my application I use Adobe Reader through the COM / ActiveX interface, to display PDF documents within my application. This has been working fine for years.

Now that I have updated my application to 64-bit mode, I notice the following problem on some computers: Adobe Reader gets stuck while displaying a message "Initializing" with a blue progress bar. See the screenshot below. It just stops working here.

The funny thing is that this problem occurs only on some computers, not on all. Also when I run my application in 32-bit mode, it works without a problem.

I have checked the preferences: "Run in AppContainer" and "Start in protected mode" are both unchecked.

I would be really happy if someone knows a solution to this problem.