One possibility is some data corruption on the disk. Try moving the video file to a different hard drive.
... & preferably a local one at that. This can also potentially happen on flaky external drive connections.
Beyond that it could also be corrupted data within your media file. I've seen several satellite transport streams that were completely munged to the point where some of the data is unextractable. What's your source file?
I will try the file on the local hard drive when I get had chance, but I highly doubt that is the problem because I have had the same issue when using a file on the internal hard drive. And if it was a flaky connection it would work at some point I would assume, but it has probably tried to conform it 50x and it has always fail, and no smaller files fail.
It is a long file, actually a download of the state of the union speech from the White House. Premiere is able to play the file without the conform.
Since you say the process takes hours... I would suggest using AME and Encoding the file to a more stable CoDec that you know plays well with Premiere. That way you don't have to go through the process but once.
Set it and let it run over night.
Wake, Bake, and Edit Happily!
If AME has a problem with it, download the 30 day trial of Vegas, load the file in there, and export to an edit friendly CoDec.
Not trying to endorse a competitor so substitute your favorite NLE or Encoder for Vegas.