I cannot see anything unusual in the log. I updated to calibre 5.20 and it is working fine for me.
Maybe something is wrong with the plugin's settings. They are library specific and are stored inside the database so are not easily removed.
You can try to create a new library and make couple of empty identical books and see whether the plugin works fine in the new library.
Note: The plugin settings can be removed if you have can open the customize plugin dialog. But you must replace the settings with curly braces like below:
It is better to try the new library thing first before going down this route. If a new library does not make this go away, there is no benefit in removing the settings.