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When only LO is open, but no documents are open, icons of previously opened documents are shown. Often when clicking on those icons I would have the same problem. However, clicking on the open file menu and navigating to the location of the actual document instead solved the problem Apparently the links from the icons shown in LO itself are getting unlinked (disconnected) from the actual document locations. When that happens LO can’t find the document. You have to find it for LO.