This can happen, if the DXF file is somehow locked, e.g. by another software.
I have this sometimes, when the DXF is still open, while it is imported in OpenSCAD.
Then OpenSCAD locks the DXF and changes in LibreCAD can't be saved in the original DXF.
When saving fails in LibreCAD, it opens the
Save as dialog to not lose any work.
I also remember issues, not in LibreCAD, but other office software, where saving failed when the file is located on a network drive. The consequence in LibreCAD would be
Save as too.
So this has probably nothing to do with print preview or its settings.
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