Repro steps:
1. Set up a Vagrant Windows 10 Virtual Box
2. Create a shared folder, accessible by both the host and the guest machine
3. Create a Unity project on the Desktop of your guest machine
4. Move the newly created project into the shared folder
5. Launch the project in Virtual Box

Expected results: Unity Editor launches and the project is opened
Actual results: Unity Editor doesn't launch, the project is not opened

Reproducible with: 2017.4.1f1, 2017.4.34f1, 2018.4.0f1, 2018.4.13f1
Not reproducible with: 2019.2.0f1, 2019.2.15f1, 2019.3.0f1
Could not test with: 2020.1 and all alpha/beta versions due to inability to install Unity Hub on the VM

Notes:
Make sure that the guest machine accesses the shared folder via Network (the path should be \\VBOXSVR\<Shared Folder>)
While creating projects, the path with no disk in it is deemed invalid, even if it was chosen by browsing
*Opening the project in 2019.2 and 2019.3 throws additional pop-up messages compared to when the same project has the disk in its path (you can get the same messages by launching these Editors with no Hub installed)