Apple - El Capitan: Appstore says "downloaded" and "there's been an error" (stuck)

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.

me too. On my 2009 iMac, found it under Install OS X El Capitan in the Applications folder. Wasn't sure what was happening as it seemed to be stuck on Downloaded on the Mac App Store, and re-starting my computer wasn't doing anything. Seems to be running now