Browser sync storage is limited to ~100KB shared across all data (identities, permissions, relays, and Cashu tokens).
If you plan to use the Cashu ecash wallet with significant balances, choose "Sync OFF" which provides ~5MB of local storage (enough for ~18,000+ tokens vs ~300-400 with sync).
Note: Cashu tokens are bearer assets. If you lose your vault backup, you lose your tokens permanently. Make sure to configure regular backups.