Error when rejecting take offline request

Also applicable to "Schedule take offline (request)".

To reproduce:

• Log in as author
• Open a document which is live
• Choose "Publication" and then "Take offline (request)..."
• Log out
• Log in as editor
• Open the same document
• Choose "Requests" and "Reject request"
• In the dialog that opens, enter some feedback
• Click "OK"

Expected behavior: The dialog closes, the document is taken offline and the feedback can be read by the author
Actual behavior: The dialog remains open and displays an error: "An error has occurred" (see screenshot)