I have exactly the same problem running 1.6 . After installation and creation of a menu button to a monthly calendar. When clicking the button I get this error.
I installed the mods and can get a small calendar in a module but when I click on a date in it I get the same error. I have made sure there is a category etc...
A little lost.
PS I have a 1.5x install running perfectly on the last version so I know your software is very nice