This error happened, in this case, during the update process from Magento 220.127.116.11 to Magento 18.104.22.168, because Magento compilation was still enabled while cleaning the Magento cache on the command line. There is a report of a similar case.
Deactivating Magento compilation solves the issue only temporarily. It re-appears when re-enabling it, with or without re-compiling all the files in between. So it appears that this is a bug in Magento core. Until it is finally solved by the Magento folks, the simplest and most permanent solution is doing this via ssh:
ln -s ../downloader/ downloader;
This solution and its further discussion can be found in this magento forum message.