internal error (in is/internal) type does not exist

interface(version);

Physics:-Version()

restart;

eqs:={0 = ln(2)+ln(-a*sech(1/2*2^(1/2)*b)^2), 1 = -tanh(1/2*2^(1/2)*b)*a^(1/2)*2^(1/2)};
unknown:={a, b};

solve(eqs,unknown)

solve(eqs,unknown)

restart;

eqs:=[0 = ln(2)+ln(-a*sech(1/2*2^(1/2)*b)^2), 1 = -tanh(1/2*2^(1/2)*b)*a^(1/2)*2^(1/2)];
unknown:={a, b};

solve(eqs,unknown)

restart;

eqs:=[0 = ln(2)+ln(-a*sech(1/2*2^(1/2)*b)^2), 1 = -tanh(1/2*2^(1/2)*b)*a^(1/2)*2^(1/2)];
unknown:=[a, b];

solve(eqs,unknown)