Package: pari-stable
Version: 2.7.5, 2.7.4

It appears that the infinite loop (or similar) is entered, 
but only when modulo is prime p and isprime(p\2)

Example:
p = 268890659;
znlog(Mod(11,p),Mod(10,p))
#-- hangs

Same with order parameter:
p = 268890659;
t = Mod(10,p); 
o = znorder(t); \\ o = p-1, anyway
znlog(t+1,t,o)
#-- hangs

The bug is not present for p < 2^28 or when !isprime(p\2).

Serge Batalov