If Kdenlive or one of the Piave renderers crashes, it is possible that it will prevent the current or and new copies of Kdenlive from running correctly.
There are two reasons why this may happen. The first is if one of the Piave processes did not shut down on the previous run of Kdenlive. The simple solution to this is to close down Kdenlive and type from the command line:
% killall -9 piave
This will kill any stray Piave processes, and should allow you to run Kdenlive normally again.
If you try the above and it says “piave:no process killed” then it may be that one of the ports that Piave listens on has not yet been freed after the previous run. Wait for about one minute before starting Kdenlive again - the TCP port will time out and become usable again.