Hi Hans, I have found this on one or two occasions. I don't know what caused it but I certainly did not want my other pseudobulbs to be infected.
Case 1) this looked like yours and I threw it away instantly.
Case 2) here the brown patch was quite small. This is what I did:
- cut it out with a very sharp knife right down to the pure green flesh,
- dusted it with sulfur powder (I could have used nutmeg, it would have had the same effect)
- let it dry,
- put it in a separate container and
- planted it individually the next spring.
No further problems occurred.
Generally, I discard rather than saddle myself with a lot of individual treatment.