Hey Mark,
...What was "the use" refered to that you found in CONFIG.SYS, related to the other versions you had installed, out of curiosity?
I am using the standard Arca Noae package manager RPM versions, currently installed for 5.0.1 (libcx 0.6.0-1). Where did you get the libcx 0.6.1-2.oc00 version?
If I have a link, I will download and see if it solves the problem here for the 2018, v5.0.51 of VirtualBox as well, as the previous posts on this thread also seem to relate to LIBCX in some way, shape or form.
Ahh..."the use" was simply reflecting the fact that I am using the 'VBOX_USER_HOME' variable to point to the VBox home, and previously with different versions I had them based in the program directory instead of the system 'HOME' location. Sorry, I think that caused some confusion...I've attached my CONFIG.SYS so you can take a look at it. Hopefully it'll help you out answering the question regarding the location of the driver as well.
The key to making my install work was to point to the corrent driver...I was not using the matching release and that was enough here to cause exactly the same problem you are seeing.
Regarding libcx...I did move to the test release, you can get it from the netlabs-exp repo, again, take a look at the attached file, needs to go into \etc\yum\repos.d directory, but read-up on it since I'm not sure how much you know about the YUM/RPM setups. The general concensus is that it is NOT advisable to install the stuff from non-release repos, so YMMV!
EDIT
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libcx specifics...OK, so if you pick the latest netlabs-exp version you will need to run vBox with the 'SET LIBCX_HIGHMEM=2' in either your CONFIG.SYS or if you put it in something like vbox.cmd (see attached) file it will only affect vbox app instead of everything on your system that uses libcx. Here is a link (
https://github.com/bitwiseworks/libcx) to the github for libcx that explains this well (look towards the bottom of the page =>
https://github.com/bitwiseworks/libcx#notes-on-c-heap-displacement)
I am curious if vBox could be re-build to make use of the modified version of libcx to avoid having to do this...far too complex for me to understand though...LOL