You can't import private keys into an Electrum 2.x wallet with seed. If you want to import to Electrum, you will have to create a new wallet, select 'Restore wallet' and then enter private key(s).
https://electrum.orain.org/wiki/Frequently_Asked_Questions#Can_I_import_private_keys_from_other_Bitcoin_clients.3F
for the easy ways, is to import your privatekey to blockchain.info then move all your bitcoin into your wallet
You can also try to recover it with other wallet or blockchain.info
Agree, I've also had this problem before on an earlier version of Electrum. I was however able to import it into my Blockchain wallet so did that and just moved the funds back to Electrum.
Was most probably something related to the format of the private key.
I highly recommend not to import private keys to Blockchain.info or other online wallets, if you are going to use that addresses. Anyway, as you can import private keys to Electrum, I hope you won't import it to an online wallet.