Originally Posted by olandese
i did a little more investigation and the problem occours only with .chm files. ...
It seems to be that the file is still in use.
I've confirmed the issue and you are dead right, the chm input reader plugin is not releasing a file handle. I'll add a report to the bug tracker, I've isolated it down enough to prove a fix that "works" but Kovid or someone will need to put it in the right place. So it will require a Calibre release to get the plugin to allow you to open CHM files without restarts.