Thanks for your work on CbB Noel, and I was afraid this was the issue. But everywhere I saw to download BandLab Assistant, it says it's for "Windows 7 and higher" (see attached screenshot, taken directly from https://www.bandlab.com/products/cakewalk).
Did BandLab notify current Win7 users that the next version would not be supported? If so, I didn't get anything about it, and it appears other people didn't either. (Yes, the install screen on the last couple of versions said that Win7 wasn't supported -- but the product still installs and everything works fine.)
I've used Cakewalk/Sonar since the DOS days, and I spent a few years helping out in the old forums (I still help users on KVRAudio.com and Gearspace.com). This was the first time I had to use the new forum, and I appreciate the help and the link to the fix. That said, it seems CbB (and BandLab Assistant) simply could notify the user that the OS in use is no longer supported, or make this a "sticky" for the forum.
For what it's worth, I looked through the forum a bit for an answer to this. But while the first issue posted in the thread you sent me had a similar issue as mine, the user was on Win10 -- and no one posted a solution. I didn't go through 5 more pages of the thread to see the link to the Microsoft fix. And while I got this same error applying the previous update (Build 50), it actually installed, even without me doing anything different.
Thanks again for the help, everyone. I'll be upgrading to Win10 sooner than I'd hoped, but at least I can get another week or two out of Cakewalk by BandLab!