My company uses a different file structure, which includes some code to have a second plugins directory as well as a differently-named content directory. There are many business reasons for this, which include the ability to have a common set of plugins in a git submodule.
Anyway, because of this, when we try to put plugins in this second plugin directory we can sometimes see hiccups in the way they call their individual files - like css/js/other php's in "require()" statements, etc. For the URL's in Simple History, if you change the way it looks for its styles/scripts to be relative to the main script file these bugs won't happen. I believe this might help with multisite users as well (although I haven't tested this.)
I made this change to line #33
define( "SIMPLE_HISTORY_URL", plugins_url('',__FILE__).'/'); // /relative/url/to/simple-history/folder
Hope this helps. Thanks for a great plugin.