It's hard to work out how this happens from the info you've provided.
I've not seen a NaN come about as the result of integer overflow as Laurent_R has suggested ... but I don't have an alternative explanation.

Given that at 20::56::12 "Rollover Transferred Till now" was "NaN bytes", it stands to reason that every subsequent "Rollover - Transferred Till now" will also be "NaN bytes" since NaN + X is always NaN (where X can be any positive, negative, zero, infinite or NaN value).
So it's probably just a matter of working out how that NaN value gets there in the first place.
And I can't tell much about that from here. Do you have a small demo script that exhibits the behaviour ?

Cheers,
Rob