Thank you to those who have replied.
I forgot to mention this is 2016 version.
Subramaniam, I think if you re-read my original post, you will find the answers to your questions. The WorkingSetMaximum is not in the config file by default but needs to be added if the server is running additional applications besides SSRS. In this configuration, SSRS has all of the available memory to utilize, which, in my case, is over 50 GB. As mentioned, I did set the WorkingSetMaximum to 50 GB and still am receiving the same error. I set the MemorySafetyMargin to 80 and the MemoryThreshold to 50 to give more resource if the process moves into a high memory pressure scenario, which I would assume by the nature of the error, is. I am currently working with my report designer to lessen the complexity. I'll let you know how that goes.
Please do post back. Curious as to what changes you see in the Execution log compared to the earlier executions.
That is one workaround that often corrects the issue you see - just add that extra piece to the query and it pushes it over the edge. But nice job of tweaking with the memory settings earlier. Something not understood by too many 🙂
Sue