Is it safe to reuse the same random session number r in the Guillou-Quisquarter protocol?
Is this safe?


No, it is not.  Here is how it would be easy to recover $A$:


The prover generates $r$ and publishes $T$, and the verifier selects an arbitrary challenge $d$.  The prover responds with $T = A^d \cdot r$
The prover generates the same $r$ and publishes the same $T$.  The verifier selects the next $d' = d+1$ as his challenge, and the prover responds with $T' = A^{d'} \cdot r$


The verifier can then compute $T' \cdot T^{-1} = (A^{d+1} \cdot r) / (A^d \cdot r) = A$, thus recovering the secret.


  I was not able to find indications in the literature about it and r is never defined as a nonce but I am afraid this could leak the provers secret key A.


Actually, it's not a nonce; calling something a nonce implies that the only requirement it has is that values never repeat,  In this case, you can devise ways to exploit related $r$'s as well; for example, if it's a simple incrementing pattern, that is, $r' = r+1$, then the attacker just selects $d = d' = 1$.  There might be nonrandom update patterns that are safe, but why risk it?

In general, using nonrandom challenges (either repeating or related) within a zero knowledge protocol does leak the secret.