I've checked now, but all the indicators definitely work with LookBack=0. Lookback=0 just switches off the error message.
So your problem has a different reason that is unrelated to LookBack. Without seeing your code this is of course hard to tell. It's not reversing the series - this just changes the sign of the slope.
Some lines of code for testing LinearRegSlope wit no LookBack - maybe it helps:
void run()
{
LookBack = 40;
vars Prices = series(price(),40);
if(Bar > 40) {
LookBack = 0;
printf("nSlope = %.6f",LinearRegSlope(Prices,40));
LookBack = 40;
}
}