Hey guys, so I am by no means an expert on any of this stuff, but I was having the same issue, all of a sudden windows started popping up saying "Keychain Not Found" when I went into Keychain Access my login keychain was an empty square (no lock icon) and when it was selected it said it was "read only" I had no luck with the keychain repair. The most annoying part is my iCloud login would pop up constantly, and when I attempted to sign in it would create a cascade of "Keychain Not Found" errors. When I went into Keychain preferences and attempted to reset my default keychain it gave me the "[UNIX]Invalid Argument" window. So... Here's how I fixed it.
FYI I think my fix will dump all of your saved passwords and what not, so try some of these other fixes first, but ultimately I was so frustrated I could care less about losing some passwords.
1) Go to Keychain Access
2) Select login keychain in the upper left corner
3) Select file > delete Keychain "login"
4) When the window pops up asking if you want to "Delete References & File" or just "Delete References" select "Delete References & File"
5) Restart your computer
6) Open Keychain Access, during the restart your mac should have created a new "login" keychain.
I tried most of these other fixes, and a bunch off of other websites but nothing worked, so last ditch effort, worked like champ, no more messages. Good luck!