I'm not sure why you would be getting this error if it was working correctly before. The file referenced is usually found in the kfm folder inside of includes > pear. I'm including a copy of it here for you in case you can't find it. You should check to see if it is in the following location:
HTMLEditor\editor\plugins\kfm\includes\pear
If it is not then you should add it here. If you can find it here then you may need to copy this file to the same location as the file:
HTMLEditor\editor\plugins\kfm
Let us know how it works for you.