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Hi Mike,

1) Where do you save your drawings? Locally or on the Network drive? If the second, then: can you reproduce this issue locally?

2) Can you attach one of the drawings you "Save as" from AutoCAD LT 2017 and then unable to open in 2015 version of AutoCAD ? You can delete everything in this drawing before saving.

3) There should be no issues between AutoCAD and AutoCAD LT if you are using the same file format.