Firstly, in order to save the content of the file , export the file as the IDML.
Thereafter close InDesign and re-open it to check if you are able to replicate the issue again.
When I tried to export this as an IDML I discovered that my target disk had somehow become locked/inaccessible. When I just saved the file to another disk, it seemed to save just fine. I suppose this was the problem all along. I'll have to investigate that further. Thanks for the prompt!