On April 1st, the Marlin Team released a new RC for the next big release of the Marlin firmware version 1.1. After updating the configuration files (due to some changes in variable names and such things) to the current version, the flashing of the firmware was as always no problem. The fixes are listed on GitHub.
I updated my supplied example configurations on GitHub for the Kossel XL in hope that it helps someone configuring their 3d printer. The pull request is currently open. 😉