After Effects CS6 - mp4 file error

Initially I struggled to get After Effects to import footage that was shot with a Sony F3 (mp4 codec)

I got it right eventually, don’t know how but it worked fine for more than a month.

This afternoon however it just refused to work and I get the standard error saying the file is damaged or unsupported.

I checked the forum but can’t seem to find a solution the problem. A couple of people suggested that the footage be converted to another format.

So I have 2 questions:

1. Is this a bug in CS6?

2. What other format can I convert the footage to without losing any quality that will play in After Effects.

Regards

IvanRisingStar