If you only have a task collection defined within zanshin and no NoteCollection, you can't create task on the fly to the inbox or any context , tag. 

KJob *TaskRepository::createItem(const Item &item)  
 -> if (defaultCollection.isValid()) return false

I didn't dig more, but as soon as I Filled the note collection, it worked again.
Need some test on this I guess 

Reproducible: Always

Steps to Reproduce:
1. Take of the default Note collection
2. try create a task in the inbox or on any context/tag (on the fly, no drag and drop)
3. Nothing appear

Actual Results:  
No task created

Expected Results:  
Should create a task