We give an explicit dimension formula for the spaces of vector valued Siegel cusp forms of degree two with respect to a certain kind of arithmetic subgroups of the non-split Q-forms of Sp(2,R).
"An explicit dimension formula for Siegel cusp forms with respect to the non-split symplectic groups." J. Math. Soc. Japan 63 (4) 1263 - 1310, October, 2011. https://doi.org/10.2969/jmsj/06341263