I just tested with the copy of the report from a content pack, after adding Visuals, the save works at my side:
Please take a try with the Save as under File at the top-left side, see if it would help:
If issue still insists, please generate a network trace either with F12 developer tool or Fiddler:
Capturing additional diagnostic information for Power BI
Fiddler Web Debugger Tool
Any further help needed, please feel free to post back.