The spell Expeditious Retreat requires concentration, and reads:
When you cast this spell, and then as a bonus action on each of your turns until the spell ends, you can take the Dash action.
Assume that a character has cast Expeditious Retreat on themselves in a previous turn. This turn, they have used all of their normal movement, and that character starts the Bonus Action Dash and uses some but not all of the movement granted by this Dash. If concentration is lost at that point (either due to an opportunity attack/readied action or the casting of another concentration spell) is that character permitted to continue moving the remainder of their dash distance?
You can continue the movement after losing concentration
Expeditious Retreat allows you to take the Dash action as a bonus action.
At the point at which they take the Dash action they 'gain extra movement for their current turn'.
If their concentration on Expeditious Retreat is subsequently lost then they don't lose this movement retroactively.
They've already taken the Dash action and gained the extra movement for that turn - so concentration on the spell is no longer important. As long as they have unused movement (and they do) then they can still move. Maintaining concentration on the spell would simply allow them to benefit from it on future turns too.