I have just installed Virtual Box version 5.2.14 r123301 (Qt5.6.2), including the VirtualBox Extension Pack, on my Windows 10 Pro machine.
The guest OS is Ubuntu 16.04 LTS with Guest Additions installed.
When I start the virtual machine, everything is fine. I can resize the screen as I want. I can also move the window on the screen without any problems.
Problems start after resizing or moving the window a few times. Sometimes even after the first resizing.
Then I cannot resize any more. The size is frozen. Also, moving the window is not possible any more. Only changing to full screen mode or minimizing still works.
After restarting the virtual machine it works again for a short while.
I have seen this question: VirtualBox suddenly disabled auto-resize guest display
I am sure I didn't accidentally press Win+P and if I do so deliberately, the window size changes but still no resizing is possible.
I also found some hint that this might happen after inactivity when the guest-OS goes into sleep mode. I can rule this out as it sometimes happens after a few seconds if I resize soon after restarting.
The memory for the display is set to 128 MiB in the virtual machine settings.
First I thought it might be related to a large screen size (my screen is 3440x1440), as it happened after restoring from full screen mode or after increasing the height of the window. But it also happens when the resolution is less than 1920x1080.
Resizing the window via selecting a different display resolution from within the Ubuntu guest still works at this time but resizing via the host window is no longer possible.
Any hint is welcome.