I have no solution to suggest, just to mention that it also happens here as well, and only with the saved-as-pdf pages.
Debian testing, opera 60.0.3255.59, regular package from the repo (= not snap).
Drm content that plays with widevine is has nothing to do with chromium ffmpeg codecs. So, are you sure hoopla uses widevine for drm?
I won't make an account there so as to test it.
It seems your issue was fixed in version 60, although I can't find it on the changelog.
Opera now follows its own gtk theme for that downloads popup instead of the system's gtk theme.
They kind of "fast tracked" Opera 60 to Stable, while 61 is still Developer. Both Stable and Developer should be fixed (I know Developer is as I am posting from it), but until 61 goes to Beta you'll need to use either Stable or Developer.
Which version of which distro are you using? 32-bit Opera for Linux is getting close to two years old now, and may not be compatible with current versions of your distro.
If you reinstalled the OS, you need to provide your old OS password. It's necessary because your saved passwords are encrypted using your OS login information.