The transaction export from the Electrum wallet does not contain enough data to accurately generate a ledger. For this reason, you need to include your xPub key the first time you upload Electrum transactions. The xPub key is a public key and chain code that can be used to create descendent public keys. In other words, the xPub key allows NODE40 Balance do determine which public keys belong to your wallet.
How to export your xPub
Open your Electrum wallet and select the menu item Wallet => Master Public Keys.
A small window will display with your xPub key. Copy the xPub key and paste it into the upload dialogue at NODE40 Balance.
Is it Safe?
xPub keys can never be used to access your funds. The xPub does not contain any private key information.
You would not want to expose your xPub key to the world as it would be possible to build a profile of transactions related to a specific wallet. However, supplying your xPub key to NODE40 is safe because 1) the key is kept encrypted and never exposed to the browser, and 2) the key enables us to build a ledger for your personal accounting; which is precisely why you're usingNODE40 Balance.