[Nexus 6  Original Android Development] relaxable:
Don't know if this could give you another hint about what's going on - seems as if I can flash r64 and boot successfully - once - but on any reboot, the boot animation gets stuck and have to force back to recovery and flash an older kernel.

Here's logcat from the successful booting with r61 from what seems to be about the same point in the log where it fails with r64:

06-01 13:02:09.969 296 832 E Sensors : sns_main.c(566):Error writing to ctl sock fd 74, err 104
06-01 13:02:10.483 2043 2046 D libEGL : loaded /vendor/lib/egl/libEGL_adreno.so
06-01 13:02:10.513 2043 2046 D libEGL : loaded /vendor/lib/egl/libGLESv1_CM_adreno.so
06-01 13:02:10.534 2043 2046 D libEGL : loaded /vendor/lib/egl/libGLESv2_adreno.so
06-01 13:02:10.555 2043 2046 I Adreno : QUALCOMM build : a5b4970, If5818605d9
06-01 13:02:10.555 2043 2046 I Adreno : Build Date : 10/12/16
06-01 13:02:10.555 2043 2046 I Adreno : OpenGL ES Shader Compiler Version: XE031.09.00.04
06-01 13:02:10.555 2043 2046 I Adreno : Local Branch : N24D
06-01 13:02:10.555 2043 2046 I Adreno : Remote Branch :
06-01 13:02:10.555 2043 2046 I Adreno : Remote Branch :
06-01 13:02:10.555 2043 2046 I Adreno : Reconstruct Branch :
06-01 13:02:11.820 258 2042 D Cryptfs : unmounting /data succeeded
06-01 13:02:15.499 258 2042 D Cryptfs : Just triggered post_fs_data
06-01 13:02:15.618 258 264 D vold : e4crypt_init_user0
06-01 13:02:15.618 258 264 D vold : e4crypt_prepare_user_storage for volume null, user 0, serial 0, flags 1
06-01 13:02:15.618 258 264 D vold : Preparing: /data/system/users/0
06-01 13:02:15.619 258 264 D vold : Preparing: /data/misc/profiles/cur/0
06-01 13:02:15.620 258 264 D vold : Preparing: /data/misc/profiles/cur/0/foreign-dex
06-01 13:02:15.621 258 264 D vold : Preparing: /data/system_de/0
06-01 13:02:15.621 258 264 D vold : Preparing: /data/misc_de/0
06-01 13:02:15.622 258 264 D vold : Preparing: /data/user_de/0
06-01 13:02:15.622 258 264 D vold : e4crypt_unlock_user_key 0 serial=0 token_present=0
06-01 13:02:15.624 1 1 I vdc : 200 2059 Command succeeded

And this is similar location when it gets stuck in boot animation on r64:

06-01 22:57:31.679 295 835 E Sensors : sns_main.c(566):Error writing to ctl sock fd 74, err 104
06-01 22:57:32.221 2065 2069 D libEGL : loaded /vendor/lib/egl/libEGL_adreno.so
06-01 22:57:32.252 2065 2069 D libEGL : loaded /vendor/lib/egl/libGLESv1_CM_adreno.so
06-01 22:57:32.274 2065 2069 D libEGL : loaded /vendor/lib/egl/libGLESv2_adreno.so
06-01 22:57:32.296 2065 2069 I Adreno : QUALCOMM build : a5b4970, If5818605d9
06-01 22:57:32.296 2065 2069 I Adreno : Build Date : 10/12/16
06-01 22:57:32.296 2065 2069 I Adreno : OpenGL ES Shader Compiler Version: XE031.09.00.04
06-01 22:57:32.296 2065 2069 I Adreno : Local Branch : N24D
06-01 22:57:32.296 2065 2069 I Adreno : Remote Branch :
06-01 22:57:32.296 2065 2069 I Adreno : Remote Branch :
06-01 22:57:32.296 2065 2069 I Adreno : Reconstruct Branch :
06-01 22:57:32.527 262 2055 W Cryptfs : sending SIGKILL to processes with open files
06-01 22:57:32.606 2055 2055 W vold : type=1400 audit(0.0:27): avc: denied { read } for uid=0 name="fd" dev="proc" ino=9097 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=dir permissive=0
06-01 22:57:32.606 2055 2055 W vold : type=1400 audit(0.0:28): avc: denied { getattr } for uid=0 path="/proc/253/cwd" dev="proc" ino=24821 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=lnk_file permissive=0
06-01 22:57:32.606 2055 2055 W vold : type=1400 audit(0.0:29): avc: denied { getattr } for uid=0 path="/proc/253/root" dev="proc" ino=24822 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=lnk_file permissive=0
06-01 22:57:32.606 2055 2055 W vold : type=1400 audit(0.0:30): avc: denied { getattr } for uid=0 path="/proc/253/exe" dev="proc" ino=9105 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=lnk_file permissive=0
06-01 22:57:34.270 2088 2088 W cameraserver: type=1400 audit(0.0:31): avc: denied { getattr } for uid=1047 path="/vendor" dev="rootfs" ino=7759 scontext=u:r:cameraserver:s0 tcontext=u:object_r:rootfs:s0 tclass=lnk_file permissive=0
06-01 22:57:34.276 2087 2087 W audioserver: type=1400 audit(0.0:32): avc: denied { getattr } for uid=1041 path="/vendor" dev="rootfs" ino=7759 scontext=u:r:audioserver:s0 tcontext=u:object_r:rootfs:s0 tclass=lnk_file permissive=0
06-01 22:57:34.307 298 298 I ServiceManager: service 'drm.drmManager' died
06-01 22:57:34.309 298 298 I ServiceManager: service 'media.codec' died
06-01 22:57:34.309 298 298 I ServiceManager: service 'media.drm' died
06-01 22:57:34.311 298 298 I ServiceManager: service 'media.extractor' died
06-01 22:57:34.713 2055 2055 W vold : type=1400 audit(0.0:33): avc: denied { read } for uid=0 name="fd" dev="proc" ino=9097 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=dir permissive=0
06-01 22:57:34.713 2055 2055 W vold : type=1400 audit(0.0:34): avc: denied { getattr } for uid=0 path="/proc/253/cwd" dev="proc" ino=24821 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=lnk_file permissive=0
06-01 22:57:34.713 2055 2055 W vold : type=1400 audit(0.0:35): avc: denied { getattr } for uid=0 path="/proc/253/root" dev="proc" ino=24822 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=lnk_file permissive=0
06-01 22:57:34.713 2055 2055 W vold : type=1400 audit(0.0:36): avc: denied { getattr } for uid=0 path="/proc/253/exe" dev="proc" ino=9105 scontext=u:r:vold:s0 tcontext=u:r:logd:s0 tclass=lnk_file permissive=0
06-01 22:57:34.748 262 2055 E Cryptfs : unmounting /data failed: Device or resource busy