I read part of this frustrating discussion ... and did something different that it seems to be working (I still need to wait a bunch of hours for it to finish downloading).
From a terminal, I changed my current user for the management user
su - managementuser
then, I went to the garageband application directory and under MacOS found the executable file. Executed it.
And when asking for the library it worked.
As it seems that the safe mode works, what it is happening is that GarageBand needs super-user rights to do some tasks, in particular related with Spotlight.
In few words, this is a programming error :-)