Today I tried the thing that's suggested in the previously mentioned bug page: downgrading the package "xserver-xorg-input-evdev" to version 1:2.6.0-1ubuntu6. First I tried with apt-get:
sudo apt-get install xserver-xorg-input-evdev =1:2.6.0-1ubuntu6
That failed, because apt was unable to find the package version. Next, I downloaded the package manually from here and tried installing it with
sudo dpkg -i xserver-xorg-input-evdev_2.6.0-1ubuntu6_i386.deb
This failed too. This time because of a dependency to xorg-input-abi-12.1. I looked around and found out there's no such package, it's just a feature that was once provided by xserver-xorg-core. The current version of that package provides xorg-input-abi-12 (no .1 there), but I figured it was worth a try to use some force:
sudo dpkg -i --force-depends xserver-xorg-input-evdev_2.6.0-1ubuntu6_i386.deb
That did the trick and installed the package. The downside: the package is now marked as broken and apt keeps on trying to upgrade it to a newer version. I locked the package version with Synaptic, let's see if that's enough.
At least, after a reboot I was able to use the tablet for quite a long time without freezes. Maybe I'll settle with this for now.