Dragon cannot be respawned

Try the methods described in this comment to correct the ExitPortalLocation.

Existing end gateways are regenerated

If you respawned the ender dragon as often as you had end gateways the next end gateway will generate correctly. For example if you had one end gateway and it was regenerated, the next ones will generate correctly and you will after all have all 20 gateways.

In case you did not respawn the dragon as often as there are end gateways you will have to edit the NBT data of the world. Be careful with this method and make a backup of your world before trying it.

1. Open the level.dat file of the affected world with a NBT editor such as NBTExplorer
2. Navigate to DimensionData ➔ 1 ➔ DragonFight ➔ Gateways
3. Remove as many items (beginning at the top) as not regenerated gateways in your world exist. For example: There were 2 gateways. One was regenerated which lead you to this report. This means there is still one which was not regenerated, therefor you have to remove the first item from the list.