Here's when I killed HS3. I restored the old version of UltraLighting3 and replaced the log file with the one from the last backup (last night). All is well again.
Sadly I didn't copy the error message that UltraLighting produced over and over again several thousand times per second, but in the end the log file couldn't cope.
It was an exception to do with a "message queue", if I remember correctly. Does that make sense?