(22:22:56) gp> allocatemem()
*** Warning: new stack size = 4096000000 (3906.250 Mbytes).
(22:22:57) gp> G=galoisinit(R);
^C *** at top-level: G=galoisinit(R)
*** ^-------------
*** galoisinit: user interrupt after 4min, 19,356 ms
*** Break loop: <Return> to continue; 'break' to go back to GP prompt
break> \g4
debug = 4
break>
Time MonomorphismLift: lift to prec 1: -146348137730
Time MonomorphismLift: lift to prec 2: 25999
...
Looks like an integer overflow in the first debugging message. R is a degree-256 polynomial, I can send it if necessary,but I don't think it's polynomial-specific.