iCloud sign-in error: 'An unknown error occurred'

For a generic, numeric, unknown "error”, or simply unhelpful messages, I suggest you contact Apple.

Refer to this page for Apple Support for different countries ➔

Choose your country or region - Official Apple Support

Select your country (if you do not see your country, also look for "other" in regions; e.g., "Other Africa"), then "Choose a product". Click the "show" item under each category for more selections. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.