At first, try to check whether the latest 6.7-rc still suffers from the same problem. If it boots, it's something specific to 6.6.x, and we need to find an upstream fix. If 6.7-rc still doesn't boot, it has to be reported to the upstream. But without a proper log, it's a bit difficult to know to whom reporting. I guess you'd be asked to perform git bisect in such a case.