sometimes when backtesting the Ask-line disappears at some point for a certain amount of time while the Bid-price is still moving up and down. How could that be possible? What does it mean?
Every seller needs a buyer. It can be possible that the the Bid prices rise and Ask prices don't but only for some ticks, not for a longer time and not when the spread is very tight and the Bid price rises several pips (more than the range of the spread).
Try setting the spread to a fixed amount in the strategy tester (I'm guessing you have it set to current). Does it still happen?