Which 2 slaves (which slots)? Which slot is master at the time?

My first guess would be that an addon is conflicting with the keybind used by ISBoxer for assist, which is Backspace + some modifiers which vary depending on which character slot is active. One test would be whether those character assist for some masters but not others.

Anyway, sharing your config would help rule out any issues with how ISBoxer is set up.