Azure Machine Learning Studio - Labelled image can't be submitted but instead shows info message

Question

I have an Object Detection labelling project which requires a lot of bounding boxes to be drawn in one image, thus sometimes taking quite some time. Sometimes, when pressing the "Submit" button, a yellow message appears that reads "New tasks assigned: You can check previously submitted tasks or refresh the browser to start new tasks" (or something like that, the browser is not in English) but the displayed image does not change and the labelled image does not appear in the "Review" section of the labelling project.

One colleague stated that one of his tasks did show up in the "Review" section the next day after showing the message, but I'm worried that some labelled images may be lost.

EDIT: I looked at the Dev Console and noticed a 409 (Conflict) HTTP error when trying to submit a faulted labelled image. Maybe a hint?

Thanks in advance!