Attempt to Upgrade Legacy World to 26.1 Results In Crash

Hello,

ISSUE

I am having an issue with upgrading my legacy world (created in 2014/2015) to the new 26.1 world format. The latest version of the world is 1.21.11, and I have tried upgrading it using 26.1, 26.1.1, and the latest version, 26.1.2; I have had no success with these versions. When attempting to upgrade the world, it begins the process but consistently crashes the game when the loading bar hits approximately 85% completion when “upgrading legacy structures.” The screenshot of the crash has been attached to this post.

WORKAROUND

From my testing, removing the following files from the /data/ folder resolves the issue and allows the world to upgrade: Fortress.dat, Mansion.dat, Mineshaft.dat, Monument.dat, Stronghold.dat, Temple.dat, and Village.dat. I’m unsure if it is either one or multiple of these files that cause the issue, but removing all of them seems to have corrected the issue. However, I do not believe this is a reliable long-term fix as players should not have to manually delete world files to upgrade their world.

NOTE: I am using a purely vanilla world and client. I have no datapacks or mods within the world, and my launcher is unmodified.