Finally resolved the loading issue and the missing texture issue - the textures had downloaded as "filename.png.zip", each containing the corresponding png file. Extracting the contents of all the .zips fixed and fully resolved this issue.

While the above fixed the loading error, it caused some textures to be totally missing. Namely, the new carrier rail blocks appeared as grey or black cubes. Deleting the "default" textures folder and re-downloading via the launcher caused the loading error to reappear.

Issue was fixed by deleting the /textures folder, verifying integrity of data cache via steam and downloading that update, then downloading the latest update through the launcher.

Having this problem as well. Started happening with hotfix 0.19592, while it was working fine with version 0.19590. Swiching versions back to 0.19590 doesn't fix the problem.