This is an FYI mainly, as it's not a problem for me.
When I launch [ie click on] an MP4 from my media folder, it opens by default in Windows Media Player. Opus 12.2 then immediately crashes, every time in ~10 tests. I don't have any other audio/video file types atm to test, and there's no problem with images or any other file types [eg MS Office].
It's been a while since I played an MP4, so I can't pin the problem down to some specific update or event. I did upgrade Opus to 12.2 recently [1-2 weeks ago], and have had a couple of Win 10 1607 Pro x64 updates in that time period also. Opus has always been rock solid for me with opening any files for years.
Event Viewer shows an application error for every crash, which says "The program dopus.exe version 12.2.0.0 stopped interacting with Windows and was closed". Let me know if you want further detail from Event Viewer.
As I said, this isn't a problem as I can open MP4s from Opus just fine with either of these two players:
MPC -HC x64;
Potplayer Playback only (64-bit).
I notice they're both 64bit, whereas iirc WMP is 32bit even on a x64 system. Since my Opus is also 64bit, I wonder if there might be some mismatch there? Perhaps some codec issue...
No dumps, but you can add XML to MP4 as a DOpus crasher. I opened a few of yesterday's files in the dump temp area, only XMLs crashed DOpus--exact same crash.
This definitely is not normal as it is not happening here. We have one other similar report but only one, which suggests it is triggered by something else on the system. (If double-clicking files crashed Opus in general, we'd of course have hundreds of complaints very quickly.)
Shell Extensions can be invoked when you double-click files, so it is possible one of those on your system is crashing. ShellExView can be used to view the installed extensions and see what happens when they are disabled. It's often quickest to simply disable almost all of them (leaving the Windows ones is generally OK) and reboot as a quick test, then re-enable them if it didn't make any difference, or enable a few at a time until the problem returns if it did prevent the problem.
Antivirus (and similar security software) may also be causing the crash.
Both may exhibit different behavior in dopus.exe compared to explorer.exe.
(Also, make sure the viewer pane and metadata pane are not open, or anything like that, since they cause files to be processed when you click them, which may get confused with what happens when the second click is done to double-click and launch them. If the issue is with the click and not the double-click then the things to look into are quite different.)