Installing NPP 8.5.3 in combination with NotNull's suggestion fixed the context menu for me.
After just installing the update nothing had changed. Reboot didn't help.
So I re-registered the DLL, then restarted Explorer and Opus. Success.
regsvr32.exe /u "C:\Program Files\Notepad++\contextMenu\NppShell.dll"
regsvr32.exe "C:\Program Files\Notepad++\contextMenu\NppShell.dll"
With NPP installed with default settings or via Winget these commands should work for most people.
Running just the first command and then installing the NPP update should work as well.