What is the first character of the private key?
Try importing it without a label:
importprivkey yourPrivateKeyInWalletImportFormat
I've tried that.
I have two keys to import, one starts with 'D' the other with 'C', same error with both.
Do they end with "==" or "="? It might be encrypted.
For private keys associated with uncompressed public keys, they are 51 characters and always start with the number 5 on mainnet (9 on testnet). Private keys associated with compressed public keys are 52 characters and start with a capital L or K on mainnet (c on testnet).
https://en.bitcoin.it/wiki/Private_key