Logic Pro X fails to install sound library

After about an hour of downloading, I get the following error when installing any sound library on Logic Pro X. The computer is a MacBook Pro on Monterrey 12.7.6.

The internet is over Ethernet direct to the optical cable PoS equipment. The progress bar shows that all files have downloaded. So the problem is clearly not internet, but then what? (This sort of problem is all over the internet and has been for years, it would be SO nice if Apple could correct this error diagnostic to state what is really going on!)

Any clues would be much appreciated.