Just spent far too long on an issue with a user on Excel. The Trust Center and File Block settings in Office 2013 can be quite confusing. No matter what we did we had issues saving an older excel file format and all alerts kept pointing us back to Trust Center and File Block Settings. The mistake that we kept making was around checkbox and leaving boxes unchecked in there.
If you restore defaults, some Open checkboxes for legacy file types will be enabled. The user interface and common mode of thought would be to address the issue of allowing opening and saving of older file formats, you should enable all checkboxes. But, that would be incorrect. It drove us nuts. If you want users to open files, but have them open in protected mode with the ability to edit after a prompt and eventually save, you really want to enable the open in protected mode setting via enabling the open checkboxes, but you need to leave the save checkboxes alone. In the confusing user interface here, checking the save checkboxes at each file type actually disables the ability to save. So, to recap: checking the open checkbox allows/enables opening the file in protected mode, but checking the save checkbox does NOT allow saving the same type of file. Basically, this is counter-intuitive, but there you go. Crazy.