This error appears when the wallet is unable to sync with Starknet - due to large amount of accounts or some other network-related issue.
Argent engineers are investigating an issue and are working on the permanent solurion for it. While they do, there is a number of the workarounds you can try.
This is a workaround for occasions, when the wallet has several accounts and the above solutions didn't help. The steps are as follows:
1. Download first zip from https://github.com/argentlabs/argent-x/releases/tag/v5.3.10
2. Unpack zip on the system in some folder.
3. Load the old version using this guide: https://developer.chrome.com/docs/extensions/mv3/getstarted/#unpacked
4. Restore your wallet with the old version and your seed phrase - please make sure you have stable internet connection.
5. Confirm it succeeded (you can see balance in the newly installed extension).
6. Download newest version zip https://github.com/argentlabs/argent-x/releases/
7. Extract it into the same folder. You need to overwrite the files you created in step 1 and 2.
8. Restart Chrome.