I just tried and at first it didn't work for me either. However, after unsubscribing all mods sandbox+ was functional again. I'm guessing the problem here is that one of my mods, namely the flamethrower guitar, includes a lua script, which somehow prevents the execution of the one from sandbox+.
Edit: I just checked and in both scripts a new thread called "keycombo_thread" is created. Since the mods installed via workshop are loaded in after the ones in the saints row IV root folder, sandbox+'s thread likely gets overwritten with mine. Now fixing this shouldn't be hard on my end, but maybe potential compatibility issues like this should be prevented by the program that loads the workshop items in the first place.