In order to download El Capitan, I clicked the "download" button in the highlighted banner of the Appstore. The "busy" wheel began to spin in the upper left, and I thought to give it some time.
After a while (and rebooting) I noticed not much happened. When I took a look at the detail page of El Capitan on the Appstore, the download button was greyed out (saying "downloaded"), but on its left it says "there's been an error'.
Now I'm not sure if it's actually downloaded (rebooted to no avail) or that I have to go through the process again. Eitherway I'm stuk because of the greyed out button (I can't do anything). Is there a way to find out if there's actually an install package downloaded on my system (and if so: where can I find it?), or could this be an issue with the Appstore itself? I must say I'm downloading the package on another mac without any problem…
Best Answer
Apparently it downloaded the OS X El Capitan installation (under that name, or Install OS X El Capitan) in the Applications folder, which seems to be the download location. I could launch it from there.
Problem solved…
UPDATE: if this would occur during you Mac OSX Sierra download, the file you'd be looking for is "(Install) macOS Sierra" in the Applications folder.