Are you running the vbox packages matching your kernel (i.e not running the current packages with the lts kernel or the other way around) ?
This is the most common problem, especially with the users who claim they strictly followed the instructions from the help center. (the other common problem is people who mix things from the repository and from other sources).
Last but not least if you rarely use your Solus VM don't forget the url of the repositories where changed a while ago. Make sure they are correct (I don't think that's the problem but it doesn't cost anything to check)
