sorry for digging out that threat again, but I found out why the sketch didn't run. (or better: how to make it run)
It only works when the serial Adapter is NOT connected. Whenever I plug it in, it shows 2 rows of blocks. (except the first time, ie. right after uploading the code)
still don't know why this is not working, since the adapter is not interfering with the lcd (or is it??) but i thought it might be helpful for someone in the same situation.
thanks guys!