In Sytist the 4.4 update a new method of downloading photos in a zip file was developed and the following no longer applies for versions 4.4 and higher.
If you are getting and error message or a time out when attempting to download photos in a zip file you may need to reduce the number of photos per zip file in Settings -> Photo Settings.
The server will only be able to generate so many photos per zip depending on many different factors (server speed, cpu, memory, etc .... the photos sizes, if the photos are getting resized for the download or getting watermarked / logoed. ).
You may need to test out different amounts per zip file to see what number consistently works.