I just had a system freeze, which forced me to use the power button to shut down my computer. I was about to zip a folder and I think the crash occured when I accessed the zip context menu. The freeze caused one of my two screens (the external one) to go completely white, while the laptop screen, which had an Opus lister open, just froze, no mouse or keyboard input possible. (It is possible that my NVIDIA graphics card was involved in the freeze, as it is prone to freezes, but there was no message about a graphics card crash). Upon restart I found that the zip context menu had gone, although the Open With menu still lists the Multi-Format Archive Plugin. In Preferences, Enable Archive Context Menu is still checked, although none of the options below that are.
System freezes aren't normal, but nor is losing the context menu after they happen.
That suggests that when the freeze happens something is changing or damaging the registry. Possibly Windows is restoring a backup or resetting the registry or parts of it, or possibly something worse. (e.g. Data corruption from memory or disk hardware problems, or from CPU overheating or malfunction, or similar, could be causing all of the problems you're seeing, but it may not be anything as bad as that and could just be a driver bug.)
I don't know how the archives plugin could end up in the Open With list either, so that detail is a little strange, but probably unrelated to the other problems. The plugin is a DLL and has no way to open things directly (Opus itself has to open them). DLLs aren't usually chosen via the Open With menu, and it definitely does not add itself to the menu.
Thank you, Leo. By now I am pretty sure the NVIDIA driver is the culprit. I went to the NVIDIA forums and found several threads about similar freezes with this graphic card, often in combination with my laptop model. Sadly, no solution is offered, the NVIDIA people on the forum remain silent.
