For the benefit of anyone else reading this thread, as I mentioned in PM... it appears that at some point (I suspect since v3.0.0) the options have changed ever so slightly in Electrum when importing/restore wallets...
Standard wallets are now ONLY for HD seeds or "Master" public/private keys (ie. xpubs/ypubs/zpubs or xprvs/yprvs/zprvs etc). As per the "Standard wallet -> Use public or private keys" option:
So now, you
CANNOT import a single WIF format private key (ie. a "5", "L" or "K" Base58 encoded private key) if you select "Standard" wallet.
If your private key looks like:
5HsyUcGbVmbjpCfcjnSFV1D3GuxtfL5Jmk3ETa9L2zX4jKWe1Kj
L5GjoaFZmZAotojw7noR3qRF78FDEQsrPSLhbxZgvapxppQ5UUrG
KwVzGrfqFyrKyLVSjqYSvbv93Emh2BC1SUxrQHHm1nshTiNQ2Sre
When asked "What type of wallet you want to create?"...
DO NOT select "Standard wallet"... you
MUST select the LAST option ("Import bitcoin addresses or private keys"):
If you read the next dialog window properly... It'll say "Enter a list of Bitcoin address (this will create a watching-only wallet),
OR a list of private keys.":
This does NOT mean a list of private keys will make a watching wallet... it means you enter either a list of addresses, which will create the watching-only wallet...
OR you can enter a list of private keys (which will create a normal spending wallet, even though it is not explicitly stated... I've tested it
)