As suggested in the reply message, I cleared my browser cache, I closed the browser, and I then tried again to download the SketchUcation plugin and the bought Fredo extensions. Unfortunately, the result is the same: it's not possible.
When I try to download the SketchUcation ExtensionStore plugin, I get the following error:
Forbidden
You don't have permission to access /pluginrepo/rbz/SketchUcationTools.rbz on this server.
When I try to download the Fredo extensions, I get similar error messages as described in my initial mail. So, I'm afraid that 'yesterday's problem' reappears today, and that it's not structurally solved...
Kind regards.