It seems to be a race condition where the old file is closed before the new file is opened, and the logging attempt fails right at that time. The new file is created just fine, and from then, until the next 10MB checkpoint, all is fine. The reason I'm guessing this, is that it doesn't occur at each file rollover, just sometimes.. indicating (to me at least) that its a timing problem... I tried to get the code and figure out what its doing.. but.. well... um..not really able to figure it out :(
Maybe you've got a virus scanner that's locking your log files on you?