Element-R: "Set up key backup" button doesn't work

[richvdh]

• Start with an account with working 4S/cross-signing, but no key backup on the server. (One way to achieve this is to delete any existing key backup.)
• On "Security & Privacy" settings, under "Secure Backup", click "Set up"
• Enter 4S Security key/phrase

Observe:

In the console:

13:43:38.876 Error: End-to-end encryption disabled
    prepareKeyBackupVersion client.ts:3456
    CreateKeyBackupDialog CreateKeyBackupDialog.tsx:90

[andybalaam]

Steps to reproduce:

Add this to Element Web config and restart:

    "features": {
        "feature_rust_crypto": true
    },

• Log out and register a brand new user
• Go to Settings > Security and Privacy
• Under Secure Backup choose Set up
• Generate security key and keep a copy
• Click Delete to delete the backup
• Under Secure Backup choose Set up again
• Observe the error message "Unable to create key backup"

[BillCarsonFr]

Reopened because the fixing PR got reverted