Update: The issue disappeared completely while i was trying to get usbnetworking running again (by putting the usbnetworking installer bin and a suitable runme.sh (simply a call to usbnetworking.sh) into the usb-mass-storage root and rebooting). As that only resulted in "update didn't work, update error: 3" i'm pretty sure that this was not what resolved the issue.
Anyways, the home screen simply started to work again (of course from there i could then start usbnetworking again to carefully reduce the suspected troublemaker (-DMinimumKindletTimeoutMillis).
I was happily hacking away on my little project, copying my azw2 (with a nice little WIP app, doing some simple UI testing).
I had cvm configured in /etc/upstart/framework to be in debug mode, dont-suspend-at-start (suspend=n) and with a rather long delay before killing unresponsive apps (last part in the line where debugging is configured).
This development mode worked very nicely for a few days, but today after a few develop-deploy-test cycles, the home screen started crashing on launch, giving the "Unable to Start Application - The selected application could not be started. Please try again." message. I was connected via usbnetworking, and after a few tries to get it working again, "don't try hard, restart" came to my mind. I did so, but big mistake: of course after the "shutdown -r now" the thing came back up with usbnetworking disabled and the home screen still crashing.
Twenty-Second power-button reset doesn't help either (reboots, but home screen problem persists).
I suspect the problem might be related to my changing /etc/upstart/framework (in particular the kill-delay), but am not sure. Sometimes, the message takes a long time to appear, sometimes it doesn't appear at all.
USB Mass Storage still works nicely.
Any chance of getting usbnetworking up and running again, or of changing /etc/upstart/framework back to the original (of which i have a copy) some other way? My first thought was: just re-apply the jailbreak and start usbnetworking via the password-changing shellscript, but of course i can't play an mp3 without the homescreen