One of my PPimage processes is showing intermittent errors -

DEBUG: Watch Process Name : Pdf_and_email
INFO: Starting conversion...
INFO: Initializing PlanetPress Alambic...
INFO: Distilling file to job00ZNBQVKTYBCCDDA93ACD06.pdf...
DEBUG: %%[ Error: invalidfileaccess; OffendingCommand: file ]%%
DEBUG: Stack:
DEBUG: (w)
DEBUG: %%[ Flushing: rest of job (to end-of-file) will be ignored ]%%
DEBUG: %%[ Warning: PostScript error. No PDF file produced. ] %%
ERROR: I1007 : Error while converting to PDF : Error converting file: invalidfileaccess
ERROR: I1005 : Error while processing job; moving file to error folder:

The errors have started since upgrading to version 7.5.1 (from version 5.3) and all show the same error. The filename may change but its always a postscript attachment. I believe I am getting some kind of conflict between 2 concurrent instances of PPalambic. I had assumed that each instance of PPalambic had its own "space" to avoid conflicts. When I rerun the failed jobs they all run perfectly without error.