I had to do a clean install when that happened to me. Fortunately I finally had cable internet. But if you don’t have that luxury, it might not work for you.

In case anyone finds themselves here looking for answers… if you installed a fresh copy of your Logos app and the initial library download didn't finish properly…

MacOs specific:

Instead of just deleting the Download and Upload Manager folders in ~/library/application support/Logos4 (which may be fine depending on your situation…), you can just delete the whole Logos4 folder from ~/library/application support/, which will cause the initial dialogue to pop up, asking what portion of your library you would like to download. This was a better option for me since very little of my library was downloaded initially and I decided I didn't want the whole library downloaded, just the basic portion and whatever I specify. YMMV, of course.

Hint to get you headed in the right direction if you are on Windows:

Windows

1. Quit Logos.
2. Press Window key + R.
3. Type %localappdata% and press Go.
4. Open the Logos folder.
5. Open the Data folder and folder inside.
6. Delete the folders named DownloadManager and UpdateManager.
7. Open the ResourceManager folder.
8. Delete the Downloaded folder.
9. Start Logos with logging enabled1.
   

That is where to start looking, and I only suspect that deleting that entire Data folder would serve the same function as above, but consider carefully before doing. Uninstalling and reinstalling is a fine option that wouldn't take much, if any, longer.