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Workaround :

sage: maxima_calculus.diff(*map(lambda u:u._maxima_(), (E(beta), beta)))._sage_()
-2*beta^2*sum(X(k)^2*sin(theta(k))/(beta^2*X(k) + 1)^2, k, 1, n) + sum(X(k)*sin(theta(k))/(beta^2*X(k) + 1), k, 1, n)

i. e. :

$$-2 \, \beta^{2} {\sum_{k=1}^{n} \frac{X\left(k\right)^{2} \sin\left(\theta\left(k\right)\right)}{{\left(\beta^{2} X\left(k\right) + 1\right)}^{2}}} + {\sum_{k=1}^{n} \frac{X\left(k\right) \sin\left(\theta\left(k\right)\right)}{\beta^{2} X\left(k\right) + 1}}$$

which is, up to the order of the terms, what Maxima returns...

What Sage does :

sage: E(beta).diff(beta)
-(2*beta^2*X(k)^2*sin(theta(k))/(beta^2*X(k) + 1)^2 - X(k)*sin(theta(k))/(beta^2*X(k) + 1))*D[0](sum)(beta*X(k)*sin(theta(k))/(beta^2*X(k) + 1), k, 1, n)

contains D[0](sum)(beta*X(k)*sin(theta(k))/(beta^2*X(k) + 1), which is pure, unadulterated, two-hundred-proof nonsense...

This is a bug and should be reported as such.

HTH,