A bit of research shows that this happens with other dkms drivers too (not just the VirtualBox ones). If you have any experience with debugging shell scripts (running them using "sh -x <script name> <parameters...>" or "bash -x ..." is usually my approach) you could try debugging the /usr/bin/dkms script. The functionality we use is:
# /usr/bin/dkms remove -m $MODULE -v $VERSION --all
# /usr/bin/dkms add -m $MODULE -v $VERSION
# /usr/bin/dkms build -m $MODULE -v $VERSION
# /usr/bin/dkms install -m $MODULE -v $VERSION --force
For reference, the error message you are seeing is the following lines of /usr/bin/dkms:
[[ -r $read_conf_file ]] || die 4 $"Could not locate dkms.conf file." \
$"File: $conf does not exist."
Noting that the wrong variable is used in the second line, which rather makes sure that you will not find out the file name.
