You must import it into the same version of Spine it was exported from. You can run Spine like Spine -u 3.6.33 or enter 3.6.33 in the settings dialog and restart Spine. Then you can import and save a .spine project file. Unlike binary or JSON dat files, you can open the project file with any newer version of Spine. I highly suggest to keep your project files so you don't need to import your data again.

Here is a project file for your 300102.skel file:
http://n4te.com/x/8154-300102.spine
Note it had some warnings when importing because the data was exported without Nonessential checked. This means Spine had to make some guesses, especially for meshes. You should check to make sure everything is OK. You can do the import yourself to see the warnings.

We have 2 servers (in Europe and Japan) and the launcher should try both. For the next version of Spine we will have more servers and more sophisticated download mechanisms, but I'm afraid for now what current Spine launcher does to download it the best you can do. We rebooted one of the server. If you retry now, it may be better.

Sorry, the servers are up so I can only guess there is a problem with the routing between you and the servers. It seems unlikely though, since there are two in different locations.