(FWIW, my setup summary : C12.0.50 Pro / Windows 10 Pro 22H2 / 2 SSDs)
Stumbled upon a nasty issue, yesterday, just after having updated both the Steinberg Download Assistant (SDA) and the Steinberg Library Manager (SLM), with my administrator account. After getting back to my user one, I launched Cubase and got this :
I immediatly tried the Locate option and got this :
So, I resigned myself to choose the Ignore one and started to find a way to fix the issue : actually, there is none, neither here, in these forums (or I didn’t make an accurate search - all related topics found haven’t been of great help, I must say), nor in the Steinberg site. While scratching my head, I stopped Cubase and I launched the SLM as administrator and looked for the so-called missing file, which actually was here, at the right place (and as the screenshot above shows) :
Tried again to launch Cubase, after rebooting the computer : same issue and the presets related to the missing file (all the [GMxxx] ones) were indeed no longer presents in MediaBay, even after forcing a rescan of my two SSDs :
So, I resigned again myself and went this time in the SDA, also as administrator, and Reinstalled the Halion Sonic SE 3 - Content (3.35 Gb - more than half an hour for the process). Started Cubase again, same issue… 
But this time, I choosed the Remove option in the first error dialog window. This, to try again a reinstallation of the whole content, forcing the misbehaving file one.
… and again a reinstallation of the Content in the SDA. While I was waiting, suddenly, an idea dawned on me : has the .vstsound file been actually removed ? Nope… It was still there at the same place and I had to cut and copy it in my Documents folder with Windows explorer and using my administrator password to get rid of it. Finally, the reinstallation went to its end. Crossing my fingers, I relaunched Cubase : no more error message, yay ! Went to MediaBay again, and indeed, all my [GMxxx] presets were available again.
At the end, few questions that compose, combined together, a feature request :
- Why do we need to download 3.35 Gb of content, to reinstall a 4 Mb .vstsound file and isn’t it possible to allow us, precisely with the SLM, to download individually the .vstsound files that compose a content set, to avoid the never-ending download process ?
- How comes that the SLM doesn’t detect that there is an issue with the file involved, if Cubase detects it at launch ?
- At last, how comes that, in Steinberg support site, there isn’t a clear troubleshooting page for this kind of “.vstsound file missing” issue ?
Overall, several hours lost (searching, downloading, installing, checking, etc.). Thoughts and testimonies welcomed… 