Today when disassembling the rear wheel for inspection I found out the big drivewheel has some small cracks.
I recognize this problem from several consumer electronics stuff (like CD/DVD player drive motors) where sometime the small gear on the motor develops a small hairline crack and the gears do not mesh properly anymore.
In this case the cracks are not all the way through so it is still sitting tight but I wanted to stop the cracks from growing bigger. The smaller crack was quickly filled with industrial cyanoacrylate. To make the cyanoacrylate fill the larger gap I filled it with an aggregate (some plexiglas filings left on my workbench..) and soaked it in cyanoacrylate.
Photo later on..
I hope this way the wheel is strong enough again.