Steps to reproduce:
- deploy a process with a external task (with topic name foo)
- start an instance
- subscribe to the topic foo and set a serialized date variable
The serialization of an already serialized date value fails with following message:
- the serialization is not executed in such a case
The documentation  shows how to provide a date string as a value
By default the provided date format "31 march 1994" is not supported by the REST Api.
The example should use a supported date format .