Hi. One possible reason is that your workflow is simply to big. Usually these things start to happen at around 500 KB of workflow size.
You can download the Nintex Workflow Analyzer and get some first recommendations on best practices etc.
Please also have look at the following documents:
Best regards, Christian
Just check if any errors in your workflow as stated in the error message. A few days back in my environment, I received something similar error; that was because my server is missing the Temp folder. If you have access to SP servers, you can check if Temp folder is existing or deleted by mistakenly.