I accidentally archived a PM today. I clicked on the Archive button and found the PM. I clicked the checkmark next to the PM and then hit "Un-Archive Selected Thread". I then received a popup asking if I wanted to Archive the selected thread. Notice that it said Archive and not Un-Archive. I clicked Yes anyway, and it did nothing. The PM is stuck in the Archive.
Archive.JPG
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Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:61.0) Gecko/20100101 Firefox/61.0 (this will help us make sense of your bug report)