Remarkable gas adsorption properties of zeolitic imidazolate frameworks (ZIFs) are believed to be tightly related to a flexible nature of organic linkers in these compounds. We present a low-frequency dielectric spectroscopy study of dynamic effects in a ZIF-90 hybrid compound. Experiments of dehydrated framework reveal slow motion of the imidazolate-2-carboxyaldehyde linker in the kilohertz frequency range. Measurements of hydrated compound indicate two additional dynamic processes related to the adsorbed water molecules. These processes are assigned to the proton conductivity and relaxation of the supercooled water confined within the pores of the framework. We also study linker dynamics of dehydrated ZIF-90 in vacuum and under different gas atmospheres, revealing that the linker motion is significantly hindered by the guest molecules.