I have the exact same problem. I tried downgrading Jenkins from 1.642 -> 1.641 -> 1.640 -> 1.630 -> 1.620.
Opening Robot Framework report failed
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Looks like the log.html link is working again on 1.620. But I'am still unsure whether it was Jenkins 1.620 that did the trick or the Robot job execution. Btw. I am using Robot Framework plugin v.1.6.2.