Cannot install Virtualbox on Slackware 15

Dear all,

I am trying to install virtualbox on my Slackware 15, but so far no success. I've tried with Sbopkg, Sbotools (sboinstall), and directly downloading the files from Slackbuilds. In the Slackbuilds site, there is a README.SLACKWARE informing that I need to add some lines in /etc/rc.d/rc.local and /etc/rc.d/rc.local_shutdown, which I've done.
With the three options of installing virtualbox, the build starts returning warnings such as:

Code:/tmp/SBo/VirtualBox-6.1.44/doc/manual/en_US/UserManual.xml:6: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"And finally I get:
Code:kmk: *** [/tmp/SBo/VirtualBox-6.1.44/out/linux.amd64/release/obj/manual/xref-to-text.xsl] Deleting file '/tmp/SBo/VirtualBox-6.1.44/out/linux.amd64/release/obj/manual/xref-to-text.xsl.cat'
kmk: *** [/tmp/SBo/VirtualBox-6.1.44/doc/manual/Config.kmk:327: /tmp/SBo/VirtualBox-6.1.44/out/linux.amd64/release/obj/manual/xref-to-text.xsl] Error 6
Creating stylesheet {T}/manual/xref-to-text.xsl
kmk: *** Waiting for unfinished jobs....
CXX VBoxSDLHardened - {C}/src/VBox/Frontends/VBoxSDL/VBoxSDLHardened.cpp
CXX VBoxNetAdpCtl - {C}/src/VBox/HostDrivers/adpctl/VBoxNetAdpCtl.cpp
kmk: *** Exiting with status 2
Failures:
virtualbox: virtualbox.SlackBuild return non-zeroAnyone having similar issues? Any help would be more than appreciated.

Thanks in advance.