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On sage 6.4beta2 with singular version 3-1-6 (Dec. 2012) I see no such error. The answer returned is:

[r2s3^2 + ((-c4^2e2e3 + 2c4e2e4 + e4^2)/(c4^2e3^2 - 2c4e3e4 + e4^2))r3s3^2 + ((-e4)/(-c4e3 + e4))s3^2 + ((-c4^2e2e3 - c4e3e4)/(c4^2e3^2 - 2c4e3e4 + e4^2))r3 + (c4e3/(-c4e3 + e4))s3, am1 + ((-e4)/(-c4e3))ar2 + ((-2c4e2e4 - c4e3e4 - e4^2)/(c4^2e3^2 - c4e3e4))ar3 + ((c4e3 + e4)/(-c4e3))a
[r2*s3^2 + ((-c4^2*e2*e3 + 2*c4*e2*e4 + e4^2)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2))*r3*s3^2 + ((-e4)/(-c4*e3 + e4))*s3^2 + ((-c4^2*e2*e3 - c4*e3*e4)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2))*r3 + (c4*e3/(-c4*e3 + e4))*s3, a*m1 + ((-e4)/(-c4*e3))*a*r2 + ((-2*c4*e2*e4 - c4*e3*e4 - e4^2)/(c4^2*e3^2 - c4*e3*e4))*a*r3 + ((c4*e3 + e4)/(-c4*e3))*a + m4, bm1 + ((-e4)/(-c4e3))br2 + ((-2c4e2e4 - c4e3e4 - e4^2)/(c4^2e3^2 - c4e3e4))br3 + ((c4e3 + e4)/(-c4e3))b, bm4, ar1 + ((-1)/(-e3))ar2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))ar3 + (1/(-e3))a b*m1 + ((-e4)/(-c4*e3))*b*r2 + ((-2*c4*e2*e4 - c4*e3*e4 - e4^2)/(c4^2*e3^2 - c4*e3*e4))*b*r3 + ((c4*e3 + e4)/(-c4*e3))*b, b*m4, a*r1 + ((-1)/(-e3))*a*r2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*a*r3 + (1/(-e3))*a + r4, br1 + ((-1)/(-e3))br2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))br3 + (1/(-e3))b, m4r1 + ((-1)/(-e3))m4r2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))m4r3 - m1r4 + (e4/(-c4e3))r2r4 + ((2c4e2e4 + c4e3e4 + e4^2)/(c4^2e3^2 - c4e3e4))r3r4 + (1/(-e3))m4 + ((-c4e3 - e4)/(-c4e3))r4, b*r1 + ((-1)/(-e3))*b*r2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*b*r3 + (1/(-e3))*b, m4*r1 + ((-1)/(-e3))*m4*r2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*m4*r3 - m1*r4 + (e4/(-c4*e3))*r2*r4 + ((2*c4*e2*e4 + c4*e3*e4 + e4^2)/(c4^2*e3^2 - c4*e3*e4))*r3*r4 + (1/(-e3))*m4 + ((-c4*e3 - e4)/(-c4*e3))*r4, r2^2 + ((c4e3 - e4)/(c4e2 + e4))r2s2 + ((-e4)/(c4e2 + e4))r2 + (e4/(c4e2 + e4))s2 + (-c4e2)/(c4e2 ((c4*e3 - e4)/(c4*e2 + e4))*r2*s2 + ((-e4)/(c4*e2 + e4))*r2 + (e4/(c4*e2 + e4))*s2 + (-c4*e2)/(c4*e2 + e4), r2r3 + ((c4^2e2e3 - 2c4e2e4 - e4^2)/(c4^2e2e3 - c4e2e4 + c4e3e4 - e4^2))r3s3 + (e4/(c4e3 - e4))r3, r2*r3 + ((c4^2*e2*e3 - 2*c4*e2*e4 - e4^2)/(c4^2*e2*e3 - c4*e2*e4 + c4*e3*e4 - e4^2))*r3*s3 + (e4/(c4*e3 - e4))*r3, r3^2 + ((c4e3 - e4)/(c4e2 + e4))r3s3, br4, as1 + ((-1)/(-e3))as2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))as3 + ((2c4e2 + c4e3 + e4)/(c4e3^2 - e3e4))a ((c4*e3 - e4)/(c4*e2 + e4))*r3*s3, b*r4, a*s1 + ((-1)/(-e3))*a*s2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*a*s3 + ((2*c4*e2 + c4*e3 + e4)/(c4*e3^2 - e3*e4))*a + s4, bs1 + ((-1)/(-e3))bs2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))bs3 + ((2c4e2 + c4e3 + e4)/(c4e3^2 - e3e4))b, m4s1 + ((-1)/(-e3))m4s2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))m4s3 - m1s4 + (e4/(-c4e3))r2s4 + ((2c4e2e4 + c4e3e4 + e4^2)/(c4^2e3^2 - c4e3e4))r3s4 + ((2c4e2 + c4e3 + e4)/(c4e3^2 - e3e4))m4 + ((-c4e3 - e4)/(-c4e3))s4, r4s1 + ((-1)/(-e3))r4s2 + ((-2c4e2 - c4e3 - e4)/(c4e3^2 - e3e4))r4s3 - r1s4 + (1/(-e3))r2s4 + ((2c4e2 + c4e3 + e4)/(c4e3^2 - e3e4))r3s4 + ((2c4e2 + c4e3 + e4)/(c4e3^2 - e3e4))r4 + ((-1)/(-e3))s4, r3s2 + r2s3 + ((-2c4^2e2e3 + 4c4e2e4 + 2e4^2)/(c4^2e3^2 - 2c4e3e4 + e4^2))r3s3 + ((-2c4e2e4 - c4e3e4 - e4^2)/(c4^2e3^2 - 2c4e3e4 + e4^2))r3 + ((-e4)/(-c4e3 + e4))s3 + c4e3/(-c4e3 b*s1 + ((-1)/(-e3))*b*s2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*b*s3 + ((2*c4*e2 + c4*e3 + e4)/(c4*e3^2 - e3*e4))*b, m4*s1 + ((-1)/(-e3))*m4*s2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*m4*s3 - m1*s4 + (e4/(-c4*e3))*r2*s4 + ((2*c4*e2*e4 + c4*e3*e4 + e4^2)/(c4^2*e3^2 - c4*e3*e4))*r3*s4 + ((2*c4*e2 + c4*e3 + e4)/(c4*e3^2 - e3*e4))*m4 + ((-c4*e3 - e4)/(-c4*e3))*s4, r4*s1 + ((-1)/(-e3))*r4*s2 + ((-2*c4*e2 - c4*e3 - e4)/(c4*e3^2 - e3*e4))*r4*s3 - r1*s4 + (1/(-e3))*r2*s4 + ((2*c4*e2 + c4*e3 + e4)/(c4*e3^2 - e3*e4))*r3*s4 + ((2*c4*e2 + c4*e3 + e4)/(c4*e3^2 - e3*e4))*r4 + ((-1)/(-e3))*s4, r3*s2 + r2*s3 + ((-2*c4^2*e2*e3 + 4*c4*e2*e4 + 2*e4^2)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2))*r3*s3 + ((-2*c4*e2*e4 - c4*e3*e4 - e4^2)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2))*r3 + ((-e4)/(-c4*e3 + e4))*s3 + c4*e3/(-c4*e3 + e4), s2s3 + ((-c4^2e2e3 + 2c4e2e4 + e4^2)/(c4^2e3^2 - 2c4e3e4 + e4^2))s3^2 + ((-2c4e2e4 - c4e3e4 - e4^2)/(c4^2e3^2 - 2c4e3e4 + e4^2))s3 + (c4^2e2e3 + c4e3e4)/(c4^2e3^2 - 2c4e3e4 s2*s3 + ((-c4^2*e2*e3 + 2*c4*e2*e4 + e4^2)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2))*s3^2 + ((-2*c4*e2*e4 - c4*e3*e4 - e4^2)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2))*s3 + (c4^2*e2*e3 + c4*e3*e4)/(c4^2*e3^2 - 2*c4*e3*e4 + e4^2), bs4, b*s4, m2 + ((-e4)/c4)r2 ((-e4)/c4)*r2 + e4/c4, m3 + ((-e4)/c4)*r3]((-e4)/c4)*r3]