Hi Benjamin,
We have very occasionally seen these “system clock tampering” errors with Arm Compiler 6 licensing, and have made the compiler team aware. The M0+ is supported and I’ve just tried a build for the KL25Z so wonder if this is related.
Unfortunately there’s no obvious solution to this at the moment. As Jan points out, you could use GCC for now.
It would be very helpful if you could please send a log (help > report an issue > access log) to studio@mbed.com.
Thanks for the report,
Joe