Here are more details of what I've found. Connecting between C29 and A12, A13 the brake coils test about 30 ohms each so they are ok. Diodes D2 and D3 test well too, holding in one direction and showing half a volt in the other using my checker.
Stored codes show 226 six times, each with a 209 code too, some left, some right. The first 226 was 122 traction hours ago. There are no 52L or 52R codes stored.

Three traction hours ago there was a 240 code - slave detects problem in master?

The controller may have a problem in the brake driver. I've taken the cover off - quite pristine inside - all looks new, no burn smells. The original seller has offered to send it out for diagnosis.