Ok, tried the following:
3 zones, 13+12+7 modules: animation starts, but freezes after a few seconds
3 zones, 8+8+16 modules: same as above
3 zones, 13+12+6 modules: is functional, no freezing!!!
3 zones, 8+8+15 modules: is functional, no freezing!!!
So this seems to be a number-of-modules-issue rather than a zones issue. However, as mentioned before, the Parola_Scrolling example runs even with all 32 modules.
Power supply is 5V/26A, direcly connected to the power pins of each module, Arduino is USB-powered. Modules are 8x4 (8 PCBs with each 4 modules on it)