Since lyskom-keep-alive-timers is a buffer-local variable, lyskom-keep-alive-callback can't stop the keep-alive timer if the buffer has been killed. As a result, it keeps saying ERROR! in the mini buffer.
On an idea from Christer Ekholm, terminate the timer explicitly with a kill-buffer-hook instead. The error logging in lyskom-keep-alive-callback has been improved too, to provide more information on what went wrong if there are other error situations in the future.