Guys – Cubase 9.5 has a new video engine (actually, as of 9.0.30), the old Quicktime-based engine is DEAD an GONE. It’s best for security reasons to uninstall Quicktime from Windows BTW. Many threads discuss this.
The NEW Cubase video engine does NOT support all the same codecs and files as before, so you are likely running into something like that. It does support H264, but it needs to be in the correct container.
Personally, I’ve run into several issues as well: the performance is not as good as before, some old features are of course missing, and some old files don’t load correctly. However, after working through the codec and file issues, you’ll find that it is at least stable, and promised improvements are supposedly on the way.
The whole reason why they gave us a new video engine is because Apple stopped patching Quicktime, and it became a security risk. So virtually every other DAW has had to go through something similar in replacing any Quicktime-dependent engine components, one way or another. So Cubase just got a new engine as of 9.0.30 after a long time in development, and they have promised to bring back the missing features in the future, and eventually add new functionality. For now, it’s now a fully Steinberg engine, and you do NOT need Quicktime on your computer… and in fact, you have a security vulnerability if you leave it on there.