Was this happening only the first time you opened the template, but not on subsequent saves? For example, open Template.indd, make changes, File > Save, opens dialogue so you save as May.indd; later open May.indd, make changes, Save, and file saves normally.
if so this may mean the file was saved in an older version on InDesign. If you were to overwrite the old file it would be incompatible with the program that created it. When a save will introduce the risk of incompatibility InDesign treats the save as a Save As.
if that's not it, was this a network issue? Were you saving to a network drive? I'm no expert on networking but I have had issues saving in a network. I try to keep my work local but you know how it is. Do I really need to copy that file to my desktop, make a change, copy it back, delete file from desktop… ah, good night.
... View more