What bitcoin data should I back up? I have an Electrum bitcoin wallet on my PC. If my house burns and my PC is destroyed and the seed (written on paper) is destroyed, my bitcoins are lost. I want to prevent this situation.
What is the best I can do?


Storing the wallet file on another server (not possible with Electrum)
Export the private keys - address combinations and storing them AES256-encrypted on another server.
Storing the seed AES256-encrypted on another server. 
something else?
Since Electrum is a deterministic wallet having a copy of the seed is sufficient to get access to the coins. However if you also want to be able to restore the meta data in the wallet file such as the transaction and address labels and your contacts you need to periodically backup the wallet file too.

You can save a backup copy of your wallet file via file menu > save copy. Alternatively you can have your backup program grab the wallet file from the wallets directory.

If you're worried about your house burning down then do as @willtech says and keep a backup of your seed and/or wallet file in an off-site location. Cloud backups of your password protected wallet file are another option.

Note: Electrum doesn't store your wallet file, seed or private keys on any server.