What version of DT Register are you running? I'm assuming not the current version. If not, you need to get updated as there are no known CSV export issues in the current package.
I am running 2.7.13a and am also having this issue. I tried with just a few events early on and it worked. Now that I have all of the events loaded and people registering, I am also getting a blank export.
I've asked this in the SUPPORT a few times, and the only response I get is that I need to upgrade. I have upgraded, and I'm STILL getting a blank CSV. I need this figured out tomorrow, or my client will be VERY upset. Anyone? DT Register Team?