"Bad request" on completing user task with start form

My process has a number of steps generating a document. Then a user task has a start form that displays the document id and other data. All form fields are read-only. The task gets assigned to the right user.

All is fine and dandy until the user clicks “Complete”. Then I get a red banner across the browser saying “Bad request”. The same thing happens when admin tries to complete.

There is nothing in the log, no exception.

My question is, what can I do to discover the reason? This is a generic question for process design. It often happens to me that a process, after being started, bounces back to the start form without any explanation. The real reason can be things like I tried to assign a bad value to Due Date. I find it out by trial and error. The problem is the absence of error messages.

I work entirely in the UI application. Script tasks are all Groovy.

What can I do to overcome the muteness of the Flowable engine?

Answer and a more pointed question:

With Wireshark I found out that the problem was not the form at all. The problem was trying to assign a due date to a user task later in the flow. In Modeler I gave Due Date the value “${initduedate}” where initduedate is a variable set by a start form date picker.

More pointed question: How can I have all kinds of exceptions and HTTP 400 messages appear in the Tomcat log when running the Flowable UI Apps?


After starting the process instance the execution will continue until the first wait state by default. So when the create user task logic fails, you will see this being logged and sent back to the client application. If this is not the desired behaviour, you can also create an asynchronous task as the first task after the start event, to make sure the process instance is started and the client application receives an OK back. Then the async job executor will continue the process execution in the background.

With which call are you starting the process instance?

Best regards,


The setting is the Flowable UI apps so I don’t touch the API. The problem is: I don’t see anything in the log, i.e. the Tomcat log. I used Wireshark to discover the source of the error.