On startup, the UI never appears. The claws-mail process begins and sits in the background, but I have no way of interacting with it. I have tried gdb and valgrind. Neither one produce any results, as there does not appear to be any kind of "crash" happening. The UI simply does not appear. I am running Ubuntu 14.10 with KDE 5. Any other info I can provide I will be happy to.
remove /tmp/claws-mail-[USERID] and then start. This a result of an unclean shutdown. I've seen it, for example, when I use claws-mail via ssh and my connection dies.