Originally Posted by kiwidude
Thanks for the details. It sounds like something Kovid might have interest in on the Calibre bug tracker. My plugin just calls Calibre code to try to read the ISBN, and I would suspect that in the case of CHM files that input converter isn't releasing resources properly somehow.
I'll confirm it isn't anything to do with the plugin itself and put it on the bug tracker for you if you like.
the plugin is also much faster with pdf than with chm files.