After finally re-setting up our iMac that we haven’t used for quite a while, it unsurprisingly had a lot of updates to run. The actual OS was the most notable one, which was still Mavericks version 10.9.5.
Doing some research, I found that Big Sur version 11.x is the most up-to-date version, and you can update to the latest version from at least as far back as Mavericks. However, a change to the file systems used by the OS happened between the Sierra and High Sierra versions, and so online discussions about OS updates seeming to snag when trying to update from versions older than High Sierra to versions newer than High Sierra drew suggestions that it was best to update to High Sierra first among the versions that use the new file systems.
Fortunately, Apple does have a website with links and information for updating to versions that aren’t the most current macOS at https://support.apple.com/en-us/HT211683. The problem is that every macOS version from High Sierra through Catalina that I tried to download would return a “500 Internal Server Error” in the App Store.
I took to the internet with this error and found several different suggestions ranging from booting in Safe Mode – which I did try – to doing a complete factory reset with hard drive deletion – which I didn’t do. I did, however, reinstall Mavericks from the Recovery menu at system boot, which I thought was actually going to delete all files in the process, but didn’t. Whether this really helped anything, I can’t say, because I never could get any macOS handled through the App Store to download using Mavericks without getting the error, including after doing this OS reinstall process via the Recovery menu.
In the end, I went below High Sierra to just Sierra, which is listed as the first macOS titled version (previous versions were referred to as OS X), but which is still upgraded to via a traditional image file download. This actually solved my problem. I was able to update Mavericks to Sierra following the prior methods of OS upgrade. Once Sierra was installed, I still followed the suggestion of updating to High Sierra first before going to the latest available version.
I hope this helps someone else in this situation, because I literally spent several hours doing file backups and trying various suggested remedies before finally doing the manual upgrade using the Sierra image file as I explained, and I never once saw that suggested by anyone. Once that was done, I never received any other errors trying to update/upgrade any software from the App Store.