Thanks again for a very convenient plugin!
Regarding the "FAILED TO COUNT PAGES" errors and updating: After updating to the latest 1.3.1 (from some early version, albeit one with word count, unfortunately I replaced it and don't remember exactly) I got this error with no traceback. It turned out that my custom column had moved from Word count options to Page count options, see attachment.
I don't use page count (which now had the #words column configured), and Word count suddenly had no column configured. Of course it's easy to fix if you know about it, but there was no hint about this in the error message making it a bit confusing. I am completely sure that I didn't change anything myself.
I guess this is due to a stale configuration, in config.py from the updated plugin the keys are named
KEY_PAGES_CUSTOM_COLUMN = 'customColumn'
KEY_WORDS_CUSTOM_COLUMN = 'customColumnWords'
And this suddenly became a too long post about something that's probably a non-issue