VirtualBox Ubuntu Guest Additions not installing : modprobe vboxsf failed

Ok, so this is a known problem. Instead of using the image provided by virtualbox you should just download it trough aptitute sudo apt-get install virtualbox-guest-dkms and if you need the headers download this sudo apt-get install linux-headers-virtual instead of the classic sudo apt-get install linux-headers-$(uname -r).

Cheers

Edit: Changed the {} for () as pointed by @Abdelouahab.

open the terminal and run sudo reboot.

then try to install again via devices -> Insert Guest Additions CD Image.

worked for me.

in Ubuntu 16.04 I had to reboot the O.S. before installing virtualbox-guest-additions in order to load the correct kernel

A similar problem can occur with VirtualBox 5.2.18 and Linux guests, including Fedora 28, that are running kernel 4.18: Building the Guest Additions will fail with the same error message, "modprobe vboxsf failed", and no vboxsf module output to the /lib/modules folder.

The solution (for now) is to download and install the latest development snapshot of the Guest Additions, version 5.2.97, as explained in a ticket on the VirtualBox site.

The symptom is messages like these appearing in one of the log files written to /var/log by the build process:

/tmp/vbox.0/utils.c: In function ‘sf_init_inode’:
/tmp/vbox.0/utils.c:165:28: error: passing argument 1 of ‘sf_ftime_from_timespec’ from incompatible pointer type [-Werror=incompatible-pointer-types]
     sf_ftime_from_timespec(&inode->i_atime, &info->AccessTime);
                            ^~~~~~~~~~~~~~~
/tmp/vbox.0/utils.c:53:53: note: expected ‘struct timespec *’ but argument is of type ‘struct timespec64 *’
 static void sf_ftime_from_timespec(struct timespec *tv, RTTIMESPEC *ts)

Note the build process can output multiple log files, which means looking at only the latest one (as the message it outputs suggests) is not a reliable way of diagnosing problems. In my case, trying to install the Guest Additions on a headless Fedora server failed with /var/log/vboxadd-setup.log containing only the line

Could not find the X.Org or XFree86 Window System, skipping.

The key was to examine the file /var/log/vboxadd-setup.log.1, generated at the same time, which contained the actual error messages from the build, shown above.