All working now. Amazing!

First, I deleted that suspicious file (created in 1600) but this did not work. Then I deleted all remaining GCD files in RemoteSW folder but his this did not work either. Finally, I deleted the GUPDATE.GCD and that was it. The unit booted, took quite long to initialize the terrain database but then all was working. When I plugged the USB in it enetred MSM too.

After this I was confident enough to try to update to 5.60 using WebUpdater again. After this I had the same behaviour as before. Stuck at "LOADING..." After deleting the GUPDATE.GCD I had it working again. I thought maybe I could try using GarminCure3 and original SW downloaded from Garmin. This ended up "bricked" too. Then I realized the clock on the unit was wrong. I remembered somewhere on this forum I read that Garmin units only fully initialize once they catche position. I deleted the GUPDATE.GCD once again, got it running, went outside and let the unit acquire 3D position. Then I tried WebUpdater again and then, after another restart, it finally installed 5.60 SW.

Now it is all up and running.

A huge thanks to everyone on the forum for all the tools and knowledge. Especially to you Butters!