I see this quite frequently. Clicking that is supposed to bring up a Windows dialog allowing you to set the location for the export. PP thinks that dialog is open and won't let you do anything until it closes. The problem is the dialog doesn't actually open, and so you can't close it, effectively locking up PP with the only way out as killing the process in Task Manager (which also means you lose any unsaved work).
I don't know if this is a PP error, or a Windows error. Though I will say that it has only ever happened in the Export Settings dialog. Never happens with AME or After Effects.