Regarded to @soundlogic2236.
The other commands like /mt LexiconCategories/Entries work fine and print it in the minetweaker.txt. However, /mt LexiconPages throws this error when used. It is both when non-defined and defined ( /mt LexiconPages , /mt LexiconPages flowers).
The minetweaker.txt doesn't say much about the error, though.

Does this still occur for you in build 7?

Nope! :)