Tracker response with "invalid_indices"

I am using import_logs.py script to import IIS log files from the past. This works quite well as long as the date of an entry is not older than a certain date (180 days). The response returned by the tracker is

{"status":"success","tracked":0,"invalid":1,"invalid_indices":[0]}

When I manually increase the date of such an entry, it seems to get imported again. Are there any restrictions when tracking entries from the past?

Are there any updates on this topic?