dist_approx() not stable

Hi Constantin,

I occasionally encounter the following Error message when calling dist_approx() with:
```
da <- dist_approx(normalized_abundance_matrix)
```
Error message:
```
Error in pnorm(x, mean = rho, sd = -zeta, lower.tail = oneminus, log.p = log) :
unused argument (x)
```
This error seems to be random though: using the same script, the failure rate is roughly 40-50%.
Pls let me know if you know any fix?

Thanks,

--Kai