I had the same problem when I reduced the log to a lower sampling rate. The problem was that in one calibration point in Beast I left a blank space between words. The log combiner just inserted a TAB there so the tracer saw it as another field with missing data. There were more fields than data. Rectify the error by editing the log file and deleting the inserted tabs in field names.
This was the error I had:
"mrca.age(174-agnatha calibration)" so I deleted the tab like this:
"mrca.age(174-agnatha-calibration)"