It sounds like you have tried to do a couple of things at the same time (added a Prepare Data process and are reader extending the form during render) plus maybe setting the wrong process in the wrong field. If you get either wrong, you will get a render error reported by Workspace, but there should be a stack in the server log saying what the problem is.
Also, note that the Workbench Record and Playback function is extremely useful for finding and fixing problems in processes. In Workbench you just need to enable Recording on the processes involved (the TRS116PrepareData process and the built-in Render PDF Form process - for this last one you will need to 'get' the Process Management (system) application from the server (use Get Application under Workbench File menu)). Once recordings are enabled, use Workspace to start the process and when you get the error, use Workbench to Playback the recordings and look at the error and any data available at that time.
Now for your problem. I would suggest not doing the reader extension stuff until after you have the form rendering with the pre-populated data you need. I think you may have done the creation of the prepare data process correctly but you said something that I need to clarify. You said you created a new ActionProfile called "Prefill" which is good. But then you said "I've defined to call a separate rendering process called 'TRS116PrepareData.process'" which doesn't make sense, so hopefully just incorrect wording on your part.
For what you are doing, you have no reason to change the default Render and Submit processes provided (these are the ones specified in the Custom render and submit section). If you did change either of these, click on the Restore Default button. The only thing you are changing in this new profile is in the "Prepare Data process" section and it should have been set correctly to the 'TRS116PrepareData.process' value when you pressed the 'Create a prepare data process' button. When an asset with a prepare data process configured in the selected profile is requested to be rendered by Workspace, the prepare data process is run first and it's xml output variable is used as the input data to the render process (which merges it into the form).
The TRS116PrepareData process is preconfigured with a TaskContext input variable which will have info on the current(assigned) user (id, canonical name, display name, etc.). You could also use other orchestrations to get info from databases, the user directory, webservice calls etc. Whatever info you get needs to be injected into the xml output variable in the proper schema location for the form. This is done via xpath expressions. If the form has a defined schema file you can associate it with the xml variable to use code assist, otherwise you have to do it manually and be very careful to get it right.
Hint: To get the proper structure for the data used by the form, run a test with the Default Action Profile and use Record and Playback on the Submit process to capture the output document which is the submitted xml.
When you get this working, then try putting the Reader Extension stuff back in.
Hopefully this helps.
Thank you for your response. I've found that part of my problem was that the rendering process used to prefill the form needed to be set to "short-lived" instead of "long lived". Once I made that change, the form opens fine. I'm still trying to figure out how to get the Task Context variables on the form, though.
Strange. If you used the auto creation button to create the xxxpreparedata processs then it should have been created as short-lived.
Note that in the Presentation and Data section, when you use an action profile with a prepare data process, there is a section below the asset selection with 3 checkboxes to control when you want the prepare process to run.