Cannot respawn Ender Dragon in old World

I wanted to finaly respawn the ender dragon to get access to the new end content. However, when placing the 4 ender crystals on the portal, no dragon is spawned. Additionally, when closing and restarting the world, I have 4 new ender crystals in my inventory while the 4 I placed before are still next to the portal.

The legacy dragon check is successful:
[21:27:47] [Server thread/INFO]: Scanning for legacy world dragon fight...
[21:27:47] [Server thread/INFO]: Found that the dragon has been killed in this world already.

When placing an ender crystal the log file logs a NullPointerException:
[21:26:43] [Server thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException

My World is very old ( created in Beta 1.2 I think ) and the portal is not at 0/0 in the end, so this may be part of the issue.

P.s.: I'd rather not delete the DIM1 folder, as I have things built in the end.