I’ve been having the same or a similar problem ever since 3.4 or so, where 9 times out of 10, the tablet will not move past the Jolla/Intel logo. One thing I noticed is that you can tell when it got stuck on the logo by doing a few short presses on the power button. If it’s stuck, the backlight will switch between min and max brightness. If it’s actually booting, the backlight won’t change and you just have to wait a bit longer for the device to boot. I have no idea what causes it to get stuck so often or what lets it boot normally occasionally, as I’ve had it get stuck right after installing an OS upgrade and boot correctly on the boot afterwards, but it’s also booted correctly right after an OS upgrade and then got stuck on the subsequent boot. Connecting the USB cable while pressing the power button seems to let it boot more often than only pressing the power button, but it still gets stuck about 7 times out of 10.
I also experienced this bug a while ago, which is definitely somehow related:
I know the Jolla Tablet is soon going to be phased out, but it’s a shame to see it go out with a whimper.