It seems that I'm consistently unable to unlock an encrypted VM after resuming it.
Starting the VM always works as expected (VB asks for the password, unlocks the encrypted disk image, starts the VM, etc). However, if I suspend the VM (save the machine state) and then resume it, I get the unlock dialog (asking for the encrypted disk password), but I cannot type anything into that dialog. The CPU goes 100%, except while dragging around the password dialog (when it drops back to normal).
Moving the password dialog revealed a new surprise: it's actually 4 (yes, four!) of them, overlapping on the same spot. I can move each one of them, but none of them is able to receive keyboard input. On one of them I'm able to press 'Cancel' (buttons are frozen on the others) and that closes all 4 of them, pauses the VM and gets me to the VM close dialog (the same that you get when you close the VM window). From that dialog, I can press 'Cancel' and then the VM is in a "normal" paused state.
The only option is to close the VM without saving state (practically "pull the plug" off the VM). Then if I start it again, the disk unlock dialog works as expected and the VM boots normally.
The host is Fedora 25 with MATE desktop environment, qt5ct as the Qt5 "platform theme" and qt-gtk2 as the configured Qt5 style. The guest is Fedora 25 as well (but I guess it doesn't really matter).