I have been getting this this message when I open chrome after opening a fake Linkedin email some weeks ago. After that I can't hit a link which opens Chrome as my default browser. Because then the page tabs keep trying to open, but don't. So I put the 'sandbox' test in the command-line and have to open Chrome first from my desktop. Then I have to click the 'You are using an unsupported command-line flag:--no sandbox.' message away and it works fine. Until the next time I use my computer. But - also since that email, I keep getting an Avast message stating "malicious URL blocked". Some hidden .exe file is trying to open some shitty website! I can't track it down. Since both problems started at the same time, they must be related.
Is there anybody who has this problem? Or rather - isn't there anyone who knows a solution?

You have a virus or a nasty, thatd be my guess.
The reason your getting that message is that the the no sandbox switch is very dangerous (your basically removing ALOT of chromes security) and their saying what happens next aint their responsibility.
Have a look on googles support forums (or here somewhere) for suggestions on what to use to check for and remove the virus.
If you need to use that command to run chrome your just asking for trouble.