@cryo OK, I switched to another drive, but I think I've got the hang of it now.
First, the offending plot and nonce:
2017-06-16 18:28:48.657 INFO 8580 --- [roundPool-1] burstcoin.jminer.JMinerCommandLine : dl '285118' send (solo) [nonce '457319171']
2017-06-16 18:28:48.891 DEBUG 8580 --- [SimpleAsyncTaskExecutor-19339] burstcoin.jminer.JMinerCommandLine : strange dl source 'E:\13668371040637458609_456202240_1525760_1525760'
2017-06-16 18:28:48.907 DEBUG 8580 --- [SimpleAsyncTaskExecutor-19339] burstcoin.jminer.JMinerCommandLine : strange dl file chunks '1', parts per chunk '2', block '371829'
2017-06-16 18:28:48.907 INFO 8580 --- [SimpleAsyncTaskExecutor-19339] burstcoin.jminer.JMinerCommandLine : dl '285118' NOT confirmed! [ 3d 7h 11m 58s ]
2017-06-16 18:28:48.907 DEBUG 8580 --- [SimpleAsyncTaskExecutor-19339] burstcoin.jminer.JMinerCommandLine : strange dl result '6440749087779', calculated '285118' block '371829' nonce '457319171'

