The only situation I can envisage where lower inflation could actually be conducive to financial instability is a period of significant and unprecedented disinflation, typically in times of convergence from a state of high to a state of low inflation. Very often simultaneous financial deregulation adds new risks as well as opportunities to investment decisions.
Although a higher level of debt is sustainable with lower interest rates, unusually (and perhaps only temporarily) low nominal interest rates might give rise to the financing of too risky projects, due to the inexperience of both lenders and borrowers in a low-interest-low-inflation rate environment and due to the fact that liquidity constraints are automatically eased.