It looks like Radeon Software 21.6.1 is not compatible with PhoenixMiner 5.6d with the Radeon 6800. I've gotten reports that the 5700XT is fine with the new driver, but my 6800 keeps causing a fatal error when trying to start mining. Single GPU in the system, so nothing else to cause conflicts.
Try -clKernel 0 I have no issue with new driver with all 6800XT 6800 5700XT RX580. Wait until new version that officially support new driver then change it back to 1.