Hello,
Thank you very much for your quick responses!
@The Howling Wolves, your solution fixed my problem! I couldn't believe it was that simple.
I restored a previous system configuration (of the day before) and Windows Updates started working again.
In all my previous attempts the only thing I lost is the updates history, but I can live without that.
@niemiro, thank you very much for your help, but after restoring the configuration I re-run the System Update Readiness Tool and now CheckSUR.log shows no errors! No need to follow your instructions now.
Again, thank you very much for your preciuos help!
fralken