Hi!
As you can see from the screens, I pass a document from the interface to a start process and I can see it in the process history, but for some reason it results empty in the "variables" tab. I don't get why
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Are you having a user input task in the process after start node? I can see initially the cancel is blank ( in process history) but in variables it has value. So continue checking in history after which node execution does document becomes blank.