This anomaly applies to IC Rev. Engineering C, build codes CIAA-BA0, QFAA-BB0.
Full flash erase through CTRL-AP is not successful.
WDT is enabled.
Flash is not erased. If the device has a WDT time-out less than 1 ms and is readback-protected through UICR.APPROTECT, there is a risk of permanently preventing the erasing of the flash.
Try again.