@kevinator yes, the backfilling of this problem was finished yesterday. And all data that was missing could be back-filled.
The only exception to that is a 20min gap, from 2015-05-06 13:00 to 2015-05-06 13:20.
This data could not be backfilled, because the log that we use as a source was incomplete.
During this 20 min interval, we lost approximately 40% of events.
But the rest of the data is OK!
I will update the status in the incident report, thanks!