you need to create a special wallet, that will contain only imported keys.
see https://electrum.orain.org/wiki/Frequently_Asked_Questions#Can_I_import_addresses_from_other_Bitcoin_clients.3F
Could you guys explain the reasoning behind this? I'm going to stick with 1.9.8 for now, because I have some imported addresses that are important to me.see https://electrum.orain.org/wiki/Frequently_Asked_Questions#Can_I_import_addresses_from_other_Bitcoin_clients.3F
I haven't looked at the code but at a quick guess I'd assume this is related to the change to HD wallets/BIP32. From the answer to the item you linked, "you cannot import addresses in a wallet that has a seed. You need to create a special wallet, that does not have a seed" -- the seed creates deterministic addresses in an HD/BIP32 wallet. So to handle the import of one-off addresses they have taken the route that you need a separate, non-HD wallet to hold them.