It took me a while to figure out what could possibly be wrong.

Since I don't have any open channels in Electrum at the moment, I experimented a bit with my LND node. I have recently spent 0.00591658 BTC on a channel opening transaction. Right now, I can spend 578 053 satoshi through that channel and its reserve is 5916 satoshi. 578 053 + 5916 = 583 969 satoshi, so it turned out that I was also missing some coins. I found an additional parameter called "Commit Fee". This value tells how much money you are going to pay for the closing transaction fee if either you or the other party broadcasts the current commitment transaction (a transaction which records balances in the channel). In my case, it was equal to 13 605 satoshi. 578 053 + 13 605 = 591 658 satoshi. Great!

So, it looks like both you and the other party signed a commitment transaction with an unnecessarily high fee. I am not sure why that happened - transaction fees are extremely low at the moment. Theoretically, the commitment transaction fee should update once you send an off-chain payment, but I am afraid that the node you are connected to might still force you to sign a transaction with a huge fee.

I don't remember if you can specify the closing fee if you close the channel cooperatively. In my opinion, this would be the best option for now. I can help you choose a better node next time.