I have recently battled with Kdenlive refusing to render (24 hour struggle). The problem was I was not reading the fine print in the error message. I was using a "recent" project from the file menu.
The render abort dialogue was referring to a path & username I had used yesterday in my other distro install, that username was one character different to my username today. FAIL!
Solution: create the old user again and login to the acount PRESTO! renders succeed.
Seems it was not a Kdenlive issue - rather a unix permissions thing.
Kdenlive is addictive - keep up the hard work. :-)