m_state == STATE_DEBUG Error

I would signaltap the Nios II data master interface and figure out if it's becoming stuck (read/write issued with waitrequest stuck high). If it is stuck look at the address being issued during the access and then work your way from there. It's a bit of a pain debugging this way since it requires recompilation of the design so if you can solve it down to the point that it's just any access to a particular component causing these I would move over simulation so that you can look at everything all at once.