This issue could be related to the file structure while saving which may be getting corrupted. This has been reported as a bug for this release.
(1) Copy and paste entire contents of the main model from the old one to a new one and save that as SLX format.
(2) Save the original file in MDL format first, using "Save As" and re-save in SLX format. Although this is a convoluted method, re-saving the final SLX format as SLX again does not lead to the error later.