serial port already in use - out of the blue

Thanks everyone for the replies, but amazingly I still haven't been able to fix this. I figured a reinstall of the OS would work (good call Wimbur), but I'm really trying to avoid that if possible.

Robacarp, I'm up for trying the rxtx idea, but I'm a bit beginner with terminal stuff though. Do you mind quickly writing out how to create that directory? Thanks in advance.