Black screen after bootup

[Namusa]

Greetings all.
I've been using Ubuntu for months now without any issues. But suddenly, whenever I boot up the machine, I end up stuck on a black screen. I am, however, able to change window configurations, and I can close the machine as I always do. But the actual content is stuck on a black screen. I can see the VirtualBox bar on the bottom and interact with it. I hope this info helps guiding you towards a possible cause, though I am sharing the zipped log to provide better context. I've searched far and wide for a solution, but most solutions I could find didn't apply to my case (for example, allocating more Vram, I already have 128MB).
If anything, lately I've been getting notifications warning me about low disk space on my VM. I am not sure this is really the cause of this, because I saw these notifications quite a few times before this happened.
I'm sorry I don't have more information about the issue, but I'm in a pinch, since I can't work without Linux.
Thank you, and have a good day!

[mpack]

Well, at least one cardinal sin has been committed:

00:00:01.999396 VirtualBox VM 6.0.24 r139119 win.amd64 (Jul 9 2020 19:37:22) release log
...
00:00:11.255108 VMMDev: Guest Additions information report: Version 6.1.10 r138449 '6.1.10_Ubuntu'

You must never EVER have a software plugin that is newer than the software it is plugging into. And that's leaving aside that these GAs aren't even for VirtualBox, they're from an unsupported third party VirtualBox fork created by the Ubuntu people.

Please upgrade VirtualBox on your host to the latest version, i.e. 6.1.16.

I should also mention that you have not installed the host extension pack, so any VM that needs USB2/USB3/VRDP etc will fail.

[Namusa]

Thank you for the reply, it was a good place to start. I upgraded VirtualBox to 6.1.16 and installed the host extension pack, but the problem persisted. I tried changing the Graphic Controller, and tweaked the settings a whole bunch, and tripled the vdi size, but nothing changed.

[scottgus1]

Zip the logs then they will fit. The last log was a snip, with half of the diagnostic info missing.

Ubuntu ... stuck on a black screen ... lately I've been getting notifications warning me about low disk space on my VM.

If Ubuntu runs out of disk space it can get so it fails to log in. The same thing happens on real PCs, try googling "ubuntu ran out of disk space now wont log in". Solve by using Ubuntu's help channels and web-searches, and ignore that the "hardware" running Ubuntu is provided by Virtualbox.