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We use expansion $$\frac{z-1}{z^{a+1}-1}=\frac1{a+1}\sum_{w^{a+1}=1,w\ne 1} \frac{w}{z-w}.$$ Now use Abel-Poisson regularization of your sum, multiplying $n$-th term by $t^n$ for $t<1$: $$ \sum \frac{n-1}{n^{a+1}-1}=\frac1{a+1}\lim_{t\rightarrow 1-0} \sum_{w^{a+1}=1,w\ne 1} w\sum_n \frac{t^n}{n-w}. $$ We have $$\sum \frac{t^n}{n-w}=\int_0^1 \sum_{n=1}^{\infty} t^nz^{n-w-1}dz=t\int_0^1 \frac{z^{-w}dz}{1-tz}.$$ Hence our sum equals $$ \frac1{a+1}\int_0^1 \frac{\sum_{w^{a+1}=1,w\ne 1} wz^{-w}}{1-z}dz. $$ This may be further rewritten in several ways.