I have the same issue, migrating map and sequence to reduce the size of my project. Both projects run on 5.2.1 and this one map file doesn’t show up in the editor. File exists in the explorer, in the actual file structure. Tried the trick with creating a new level with same name, notified me that it already exists, therefore it can’t create it but nothing showed up. It is not the first time I experience this error and it’s driving me nuts… I could reduce the project by half but it refuses to work so I have to keep unnecesarily big projects. Wish there was a simple way of reducing a project like in After effects for example (you click what levels you want to keep and it reduces everything not related to them).