For anyone coming from the internet who wants another solution that Just Works:
Remove Vault From Opener List
While the vault in question is stalled, Shift+Click the Obsidian icon (or Mac: File → Open Vault) to open the vault opener. Open the Sandbox Vault and close the stalled vault. Open the vault opener again and you should be able to remove the stalled vault from the list. This cleanses the cache on Obsidian’s side without moving or deleting any files you may care about
Once done, re-add the Vault. If you use the default config folder (.obsidian) and the problem lies with a plugin, the problem will reappear and you know to rename your config folder and patch in your plugins and settings until you find a culprit causing it to freeze. If you use a different config folder name, it will reset to using .obsidian and you can start afresh.