I expounded quite extensively in one of the many previous topics on this issue. All the discussions finally led Alex (the former manager here,) to post a sticky topic:
We have found a bug that has a workaround. The “file currently locked by another user” message seen when the preview pane in windows explorer is open and the file is opened by double clicking. Some people get a “read only” version when opening.
To see the issue, you need to have the Preview pane open (which is turned off by default - which is a good thing - on Win 7, Win 8 and Win 10), Windows Explorer needs to be set to a thumbnail view, the model needs to have a thumbnail and the user needs t…
… which also led to a Knowledgebase article:SketchUp Help: Resolving “File currently locked by another user” error