Hello,
I had a similar problem, and contacted support, turns out the java applet was running out of memory. Paul asked me to do the following to confirm if this was the case.
"The most likely cause for this is a lack of client memory; generating reports to save as PDF is relatively memory intensive and usually spikes client memory. If you open the client diagnostics window (Help -> Diagnostics, or Ctrl + Shift + F7 if you have the menu bar hidden) then go into the ‘Console’ tab, there will probably be an error generated when you try to save the report about java.lang.OutOfMemoryError, or something similar.
To raise the client memory, open the project in the Designer, then go into Project Properties. Under Client -> Launching, there’s an initial and maximum client memory setting. As a caution, raising client memory above 1gB will prevent any users with 32-bit Java from launching your project."