Cinema 4D, used with a Wacom tablet, has been exhibiting some user interaction bugs in recent releases. Both Maxon and Wacom have released updates to resolve some of these bugs, but I still occasionally see quirks.
The most recent bug I’ve encountered has prevented C4D from recognizing when the pen is dragging, which makes navigating the viewport or manipulating objects or managers impossible. The problem persisted across C4D quit and relaunch cycles. A quick fix: launch Activity Monitor and Force-Quit the UserEventAgent process. It will automatically relaunch, and dragging will work again in C4D as expected.