Steam problems are the worst, primarily because 98% of the replies to a thread on steam are "I'm having this issue too, can somebody help ??".
Then there are always these answers. :D "I recently resolved my issue. I don't know how this was the fix, but changing my default browser from firefox to IE worked.....so strange."
This thread might have something useful.
https://steamcommunity.com/discussio...3949095/?ctp=3
If it doesn't start there, the latest comments are this July.
Although if I had to guess, if it's not your antivirus or something blocking it, it might just be something that can be fixed by restarting your computer or waiting a bit. Personally it makes more sense to me than all these things about how changing options in a browser affects the steam client's problem.