Hello Anthony,
Correct, I agree with you.
Given error message is generic and has many other reasons. Still if we tried to rule-out above possibilities one by one as below:
1. Document is certainly exists 
, other wise none of the instance would have generated.
2. I can see in above logs Max Conn limit reached (Error: RWI 00239) error also recorded. We made sure Webi processing server's "Max Connections" are adequate to perform operation. So, not sure if it relevant to this issue or not.
3. If we execute the script next time it works for last failed users; so permissions issue are out of the questions.
Any other possibilities or suggestions please?
Regards,
Anamika