Save button does not work with FreeMarkers forms

Description of problem:
If you use FTL forms for human task the save button does not work.

Version-Release number of selected component (if applicable):
BPM 6.0

How reproducible:

Steps to Reproduce:
1. Create process with human task and some output
2. Generate all forms
3. To be sure that the *.ftl forms will be displayed I have deleted all *.form files (maybe there is a better way, I wasn't sure)
4. Start the process, claim the task
5. FTL form is with input field is displayed. Type something into this field and hit SAVE button. Refresh the page and you can see that the input is empty.

Meanwhile, in the log, following exception can be observed:
Could not commit session: org.jbpm.services.task.exception.PermissionDeniedException: User '[UserImpl:'rtubati']' was unable to execution operation 'Start' on task id 4 due to a no 'current status' match

Full stack trace is attached.

Actual results:

SAVE button does not work with with FTL forms - value is not saved and WARN message is displayed in the log

Expected results:
SAVE button works with FTL forms - value should be stored and no warn should be present in the log

Additional info: