I moved the task to a separate workflow and that resolved the issue.

I have encountered the same issue. “Request entity too large” in a few rare cases. It seems most likely when the user enters a lot of text (often HTML tables) in the rich-text field in the start form of the workflow, and later when I assign a task near the end of the workflow, the error can happen. Seems the workflow collects more & more data and reaches a limit.

I do have some separate workflows for a few steps, but I had not yet considered that option for dealing with this error. I might try that. But can anyone explain what that data threshold limit is?