walldi1

Found the same in 18w20c on Server

i think i found a way to fix this for server.jar .

You have to copy the "ruin" folder from 18w19b into 18w20b "structure" folder.
Then copy .nbt data from both folder ( "underwater_ruin" from 18w20b + "ruin" from 18w19b ) into the other.

In the end you must have folder "underwater_ruin" and "ruin" in folder "structures" with .nbt data from both versions in both folders.

Hope you understand

I found the fix for the problem.

You have to copy the "ruin" folder from 18w19b into 18w20b "structure" folder.
Then copy .nbt data from both folder ( "underwater_ruin" from 18w20b + "ruin" from 18w19b ) into the other.

In the end you must have folder "underwater_ruin" and "ruin" in folder "structures" with .nbt data from both versions in both folders.

Hope you understand

I test the idear from @unknown but didn't change anything

18w20b the same. This time with 

• structure minecraft:ruin/ruin_warm6

• structure minecraft:ruin/ruin_warm7

• structure minecraft:ruin/big_ruin_warm5

Sorry. I don't upgrade an old World. I create a complete new world with complete new data.

In the other case you can see that anything is missing.
In my case if Server want load it instand crash