Anything that prevents you from playing the game properly. Do you have issues playing for the game, downloading it or successfully running it on your computer? Let us know here.
When I returned from 0.17.77 to 0.17.76, the blueprint was gone. Look at the blueprint file, but the file was emptied. Enabled blueprint cloud archiving, but it didn't help.
The blueprint library storage is not forward compatible, so if was updated to 0.17.77, 0.17.76 won't be able to load it. There should be blueprint-storage-backup.dat in Factorio's APPDATA folder.
74172 is about Steam not syncing game settings and the library, resulting in people re-enabling blueprint library sync on computer with empty library, which syncs the empty library to the cloud.