For some reason the ABS seem to work fine up until 100-110 mph. When I get to that speed, the light comes on and the ABS gets disabled. I normally don't have much use of the ABS, but at normal daily driving speeds it is working. Would be nice to have ABS on the Autobahn and Nürburgring...
I have changed both rear sensors (I managed to break one when taking the hub off one time), but the problem is still there. Is it most likely a sensor, or the ABS module? What could it be?

 

What size tires do you have front/rear?

 

225/45-17 front and 255/40-17 rear. 0,24% difference in circumference.

 

Same thing here.
The ABS light stays on, even after an ignition cycle but then goes away within 1 or 2 kilometers of normal driving, IOW the ABS resets itself.
You should / can pull the code with the paperclip shortcut in the (IIRC yellow) connector under the steering column method and then count how often the light blinks - its is described in the service manual.
Then you know what the issue is - sensor or something else.
My OBDII reader does not show ABS faults.

It happened before, it turned out to be a rear sensor (don't remember which one)
I changed both rear sensors (also damaged one trying to take it out to clean) and it was gone.
Now its back after a "mechanic" worked on the right front hub to replace the wheel bearing - god knows what he did....
I still have to see what code it is but I suspect it is the front right sensor.

I hope to fix it by removing the sensor and remove all the rust on the hub where the sensor is mounted.
Hopefully that willl bring the sensor a little bit closer to the toothed ring to give a better signal at high(er) speed.

I don't know if there is an ABS fault code for "Tires out of range" or something.
One would expect the ABS can handle some circumference difference due to front / rear tire wear......
Unless you have some super flex dragrace tires