The v26.0 update introduced significant changes to the game engine, which caused compatibility breaks with many community addons, particularly those modifying entities or world-gen.
### How to Resolve:
1. **Update to v26.2 Hotfix:** Mojang released a hotfix (v26.2) specifically targeting addon-related crashes. Ensure your game is updated to the latest version.
2. **Check Addon Compatibility:** Visit the addon creator’s page. If the addon hasn’t been updated for v26.0+, it may be permanently broken until the creator updates the manifest.json and script components.
3. **Safe Mode / Backup:** If your realm is crashing on boot, try removing the most recently added addon from the world settings via a backup of the world.
4. **Update Manifests:** For advanced users, updating the min_engine_version in the manifest.json to [1.26.0] sometimes bypasses simple version-check crashes.