I tried to install a large program using the software manager.

The install failed because "not enough space".

I then tried to remove unused software, but the package manager showed zero installed packages and I could not remove any. I decided to restart my tablet and now it is in a permanent reboot loop.

I suspect that /tmp or /var/tmp is full and causes a reboot as soon as GUI loads. Is there a way to start the tablet with non-graphical login instead?

Actually, there might be some package manager cache, or other storage for downloaded packages, that makes /var full...

Solved.

I connected my tablet to my desktop pc using usb. After each tablet reboot I had a generous three to five seconds to operate on the memory cards. I deleted everything that even remotely loooked like a cache or swap. After a few tries, it was a success (eventhough I never could access the built-in memory, just the internal and external mmc:s).

The tablet now boots properly.