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?