Electrum Error Message "22"
I've been reading the source code of Electrum, and it turns out that the reason why you can't find anything about this is that it's not an Electrum error-it's an error generated by the Bitcoin client, and Electrum is just passing it along.

Now, what causes the error?

There are a bunch of ways it can happen (like trying to send a coinbase transaction), but the number one way that this can happen is if you send the same transaction twice. Unfortunatly, the current implementation of sendrawtransaction doesn't really give you any information about why.

(For those that are interested, the relevant error is on line 555 of rpcrawtransaction.cpp.