first of all to be sure you have the correct Electrum wallet and not a malicious one you must verify its digital signature with the correct public key of the developer. checking your history,... is not a proof!

as for the failure to broadcast that transaction, it is simply because it is an invalid transaction and the server error is pretty clear. it is most probably because the public key provided inside the scriptsig is not the same public key that exists in the scriptpubkey (the hash of it) which is why it fails on OP_EqualVerify step.
try signing the transaction with the correct private key instead!
and if you really did that, then post the raw transaction here and i can tell you exactly what you did wrong.