Hi Hanspeter,Yes, modules that sit in a sub-frame cannot usually be positioned using Snap to Grid because, in the Rack Layout environment, the grid is 1U high and 1" across, designed for the accurate placement of products in the rack itself.
Products that accept modules have 'module targets'. To view the targets select Tools | View | Module Targets - you will see objects that look like vertical bow-ties.
Use the Snap to Object modifier (F7) to snap the module to the center of the module target.
Remember to turn Snap to Grid (F5) back on when you have finished positioning the modules.