In order to recover from a stopped DMA transfer, I wrote the pattern 0xC0 into ECSPI3_STATREG, to clean the TC and RF bits.
After this, the SPI interface was non-functional. I have not observed any data transfer (no clocks, master mode).
The error seems to be permanent. I have killed 2 boards, and have found no solution for this problem.
WARNING: do not write to the status register! Never!