Server crash 'caus Couldn't load structure minecraft:ruin/big_brick_8 +/big_cracked_8 +/big_brick_8

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

Ok, reopened the ticket.

Just got this crash and attached a crash log.

Different Ruin files, but basically the same thing.

 Server world was originally created on 18w19a.

18w20b the same. This time with 

• structure minecraft:ruin/ruin_warm6

• structure minecraft:ruin/ruin_warm7

• structure minecraft:ruin/big_ruin_warm5

the crash is unrelated to the structure files not being found, for that please go to MC-129644.

Personal experimentation is hinting to me that the crashes are in fact related to the missing structure files.  Here is why I say that:

18w20a and 18w20b client hangs on world generation in single player and will crash a server as well if one uses the seed WAITCHAIN and Large Biomes, giving comparable errors in the log about missing structure NBTs.  However, using a modified JAR file, I can generate and play in that world.  The fix?  Modify the client JAR file to contain a duplicate of the "underwater_ruin" directory just named "ruin".  I have not tested, but modifying the server jar in a similar way would, I suspect, solve the issue for a server as well.

Long story short, if the game can find the structure files, the crash does not occur.

Then it is in fact a duplicate.

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

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