Help! Time Machine Backup Greyed Out After macOS Update

Hey everyone, I need some advice about my Mac and a Time Machine backup.

System details:

• MacBook Pro 14-inch M1, 2021

• External 2.5-inch HDD, Time Machine encrypted

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What happened:

•	I was running macOS 15 normally, and DaVinci Resolve worked fine.
•	Today, I upgraded to macOS 26.
•	After the upgrade, DaVinci Resolve opened but all project files were missing.

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Trying to restore from backup:

•	I wanted to roll back to my Time Machine backup from 09:00 this morning (with MacOS 15, before the Upgrade to os26).
•	Booted into recovery and selected “Restore from Time Machine Backup”, but the backup was greyed out / disabled.
•	I wiped the Mac completely and reinstalled macOS 15, hoping the backup would work better.
•	During setup, Time Machine recognized the external drive and showed the latest backup (09:00), but it was still greyed out / not selectable.
•	Created a new user account and tried using the Migration Assistant → same problem: backup visible but still greyed out.

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Current situation:

•	Fresh macOS 15 installation.
•	New user account created.
•	Time Machine backup present, but cannot automatically restore or migrate the backup.

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Questions:

•	Could the encryption of the backup be the only reason for this?
•	Are there other reasons why Time Machine backups can appear greyed out?
•	What are the best ways to still recover my system and data without losing anything?

Thanks so much for any help! 🙏