The problem is in the guest-additions, the video driver needs to be updated for the recent ABI change in X (9 -> 10):
[ 5.900] (EE) module ABI major version (9) doesn't match the server's version (10)
[ 5.900] (II) UnloadModule: "vboxvideo"
[ 5.900] (II) Unloading vboxvideo
[ 5.901] (EE) Failed to load module "vboxvideo" (module requirement mismatch, 0)
The problem is in the guest-additions, the video driver needs to be updated for the recent ABI change in X (9 -> 10):
[ 5.900] (EE) module ABI major version (9) doesn't match the server's version (10)
[ 5.900] (II) UnloadModule: "vboxvideo"
[ 5.900] (II) Unloading vboxvideo
[ 5.901] (EE) Failed to load module "vboxvideo" (module requirement mismatch, 0)