I've just noticed that my MobiHandler doesn't work with recently released Amazon Kindle for PC (K4PC) version 1.5. If you install both my MobiHandler (any version) and K4PC 1.5 in a same PC, MobiHandler doesn't work.
It appears that Amazon changed the COM ProgID (an internal identifier used by Windows; not usually visible to users) for Kindle for PC, introducing File Association conflict with MobiHandler.
I'm working on fixing it. Sorry for inconvenience.
Alissa