I also have Peters Post Notes installed but cannot replicate your errors.
The line of code being referenced is:
// How many pages of content are there?
$ppn_total_pages = ceil($ppn_num_newest_posts / $ppn_num_notes_limit);
The ppn_num_notes_limit refers to the number of note sto display on the dashboard which can be set to zero for unlimited display. I usually have it set to 5, but tried 0 or nothing at all which resulted in 0.However, I could not produce the errors.
What version of WordPress are you running? What version of PHP is installed on your server? Have you tried troubleshooting to see if there is a plugin or theme conflict, meaning deactivating all plugins except peter's post notes and seing the error is still generated? If not, reactivate one plugin at a time and see which plugin is the culprit. If you have a lot of plugins it could be a tedious process, but it is an effective method.