I am on Mac OSX 10.14.3 and working with Virtualbox 5.2.22 and Ubuntu Desktop 16.04.6 virtual machine. In my case, I'm running Ubuntu in 32bit mode because I had trouble getting 16.04 to work in 64bit mode. I recently upgraded to kernel 4.4.0-143 using the typical Ubuntu update and then, when I restarted the VM again, I got this VBoxClient seamless error just like you see here. After trying all kinds of things, such as upgrading Virtualbox, upgrading VBox Extensions, and more -- the resolution was that I rebooted the VM, held the shift key down, and chose kernel 4.4.0-142. Then, I got the error, but was able to run:
sudo apt-get install --reinstall build-essential virtualbox-guest-utils virtualbox-guest-x11 virtualbox-guest-dkms
That, of course, would put the wrong virtualbox stuff on my VM, but was actually useful because then I could force the kernel to switch to the proper vbox extensions in my next step. So, with the command above typed, I rebooted my VM, held shift key down, and chose kernel 4.4.0-142. Now, this made the screen too small because it's not running the optimal virtualbox extensions that support proper screen resize with my Mac driver. Not a problem, though, because this was only temporary. So, I then ran this command:
This properly compiled the vbox extensions that were suitable for my computer. I rebooted the VM, held the shift key down, and chose kernel 4.4.0-142. At that point, the VBoxClient seamless error went away, and I had all my functionality I needed including screen resizing, bidirectional clipboard, mounting shared volumes and so on.
Long story short, the kernel version was at fault, was the best that I
could determine. There was something up with the 4.4.0-143 kernel that
was causing Virtualbox to choke. Rolling back to a previous kernel and forcing vbox extensions to reinstall was my fix for now.
EDIT: This evidently is a tracked bug with a proposed fix coming soon (it's Mar 19, 2019) for Ubuntu 16.04. It's in patch review mode, currently. More info: https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1818049?comments=all