Certainly it was not from a vulnerability in the Electrum wallet. Did you use a weak password or even no password at all?
What else was running on your Windows PC?
[exactly the reason why I do not store any funds on Windows (and Mac) and will never ever do that.]
Yes. It is pretty quick, the funds from Yo!Mix came in on 2023-01-09 18:51 and the whole Electrum wallet got drained on Date: 2023-01-09 21:35
My best guess is the hacker was somehow able to get my password to the wallet and transfer the funds. Probably from a keylogger which I haven't found yet (currently scanning my system).
The only apps that were running at the time were Adobe Photoshop, Telegram, Discord, and Google Chrome. Electrum wasn't even online until around 23:00 (midnight) where I immediately opened it after got notified by Coinomize team that the funds were transferred.
Also, just for everyone's info.
I am currently managing this bounty campaign wherein $4,000 USDT is (again) in my care, sitting on my Ledger wallet (Address)
I have already contacted several escrows including Hhampuz in hopes of being able to transfer these funds to them to act as a new official escrow.
I have also already contacted Coinomize and Yo!Mix so I can partially pay out my debts to them caused by the stolen funds. It isn't much but I hope they'll appreciate it. It's the start of many, I promise.
Note: I was not aware the project team had already transferred the funds, all the negotiations between me and this project team were done weeks ago before the events of today.