PDFCreator has stopped creating PDFs without any obvious reason

This most likely means your temp folder (C:\Users\<USERNAME>\AppData\Local\Temp\) has no or incorrect permissions, which prevents PDFCreator from creating temporary files needed for the conversion. Please follow these steps:

• Navigate to your temp folder, view its properties and click on the security tab. Add SYSTEM and your user to the list of groups/users and give them full access to the folder.
• Go to C:\Users\<USERNAME>\AppData\Local\Temp\PDFCreator and C:\Users\<USERNAME>\AppData\Local\Temp\Spool and do the same.

Note: You might need to take the ownership for the folders first in the folder properties under Security -> Advanced -> Owner.