Switched back to Dalvik from ART and although casting from apps like youtube still crashes the phone, casting the mobile screen to chromecast started working
Also, I finally managed to log a call with LG about this and...
Thank you for your query regarding the LG G3.
We have received several e-mails in regards to this and we are collecting reports and looking into this as of current. I would also like to thank you for providing us with the adequate amount of information to be able to log this as a potential bug that needs fixing. Hopefully we will be able to fix this in a rapid time and push an update to the phones to fix this issue.
I remember logging a call to Samsung support with what was a clear firmware bug on my Galaxy S3 (2 years ago) and spending the better part of two months trying to convince them that a factory reset doesn't help, nor sending my phone for repairs. They just refused to admit their firmware was with bugs
I was gearing up for another battle with LG this time, but surprise.
I'm sure it will take them couple of months to fix things, but at least they didn't try to fob me off. Is not all doom and gloom after all.