BDS 1.26.60.6 crashes when generating new chunks next to chunks saved in 1.26.52

After updating from 1.26.52 to 1.26.60, an existing world crashes the server (a native crash, with no error logged) within about a second of the server needing to generate new chunks next to chunks that were saved by 1.26.52.

• Areas that are already generated load and play fine. With view distance 6, I could fly around my base until new land started generating near the old land.

• Teleporting to any unexplored area crashes the server instantly.

• With the normal view distance (40), the server crashes about 1 second after spawning, because there’s unexplored land within 40 chunks of spawn.

• A new world created on 1.26.60 with the same seed, settings and add-ons generates chunks with no problems, including teleporting 2,000+ blocks away.

• It still crashes with all add-ons removed.

Steps to reproduce:

1. Take a world last saved by BDS 1.26.52.

2. Open it on BDS 1.26.60.6.

3. Join, and /tp to any unexplored area (or join with a view distance large enough to reach unexplored land).

4. The server crashes within about 1 second.

Expected result: New chunks generate normally.

Observed result: The server crashes. The log shows “Crash” followed by a crash ID, with no error before it.

Already tried, none of which helped:

• A clean install of the server into an empty folder.

• Removing every add-on, both one at a time and all together.

• Default network and chunk settings (view distance, tick distance, compression, client-side chunk generation, max threads).

• Editing LevelChunkMetaDataDictionary on a copy: setting NeighborAwareBlockUpgradeVersion from 4 to 6 and the OriginalBaseGameVersion/LastSavedBaseGameVersion stamps from 1.26.52 to 1.26.60.

World data notes:

• All saved chunks are version 42, with 3D biomes and subchunk format 9.

• LevelChunkMetaDataDictionary has 4 entries with NeighborAwareBlockUpgradeVersion 4 and OriginalBaseGameVersion 1.26.52. A fresh 1.26.60 world has 6 and 1.26.60.

• No experiments are enabled. The generator is infinite (type 1).

• The world includes custom biomes (vanilla_biomes_expansion:*) and custom blocks from add-ons. It still crashes with those add-ons removed.

CrashReporter Key: 8c4937c1-64cb-3532-a8dc-1deb28f67293

Crash IDs:

• c1a2a745-09f1-4531-bf9b-3f1727f0049a (no add-ons loaded)

• 18135b99-14d7-4c5b-81ea-f786ff24a2b4

• 3c40b1a6-e22c-412f-9e82-9b05e58066d0

• 16cc9233-5fcd-49a8-8fdb-f874f0f58001

• bad83658-1613-4b29-ac99-3fb19623bbac

Possibly related, separate issue: With no packs loaded, joining over LAN fails with “An error has occurred” and the server never logs a connection. Joining the same server by IP works.