Nonvanishing conjecture for the normalized intertwining operator $M^*_{c,a}$

Let $M^*_{c,a}(s,\tau,\sigma_{\bar{r}})$ be the normalized intertwining operator introduced above, with $s\in\mathbb{C}$. Nonvanishing conjecture. The operator $M^*_{c,a}(s,\tau,\sigma_{\bar{r}})$ is always non-zero for every $s\in\mathbb{C}$. This conjecture extends the preceding nonvanishing result for $M^*_c(s,\tau,\sigma)$ to the operator involving the constituent indexed by $a$ and $\bar{r}$; the supplied text gives no resolution of this expectation.