confirming, you get the message pop up: ‘There was a problem adding [unknown] to your Friends list’
example link steam://friends/add/76561198001984619
Yes, I get this issue as well with that error message.
This will be fixed in a future client release (definitely not the next one, but after that I'm not sure what release it will make it into).
I think yesterday's beta client has the fix for this. Do you still see it after updating?
Adding from URL works for me after last beta update!
Nothing extracted yet.
In steam windows, you can add friends from links like this one :
steam://friends/add/IDNUMBER
In steam linux, the only way you can add friends is by going to their profile and clicking "add friend". Many third-party websites that feature steam trading are harder to use since you can't add friends in one click...