I had a wiring mistake on a sure servo communcating via Modbus to a 260. Found the problem, but I noticed a behavior in the error response words that may indicate a bug. The lower word was fine, but the upper word didn't seem correct.
From what I understand, in each word, those bytes are supposed to be swapped. Where in the high word,the upper byte is supposed to be the CRC and the lower byte is the fault number. The upper byte of the word remained at 08 The lower byte which is supposed to be the fault kept changing with the number of words. You might want to take a look at that.