Hi @dsl145,
Thanks for posting! Unfortunately, I'm not too sure what is causing this to happen - I'm going to need a little more information from you to help me troubleshoot this.
Would you be able to send me your diagnostic log files? To collect these, launch Fusion and click the "?" in the upper right corner. Then, from the drop down select "Diagnostic Log Files" as shown below.

One thing you could try would be to save the design while working offline. This will create an item in your local cache that will be uploaded to your project the next time that Fusion goes online. Once the design saves in offline mode, could you try toggling back online to see if that pushes the save through to your project on A360?
One last thing - are you experiencing this behavior with any other design or is it just this one that you are unable to save?
Thanks in advance for the information! Please let me know if you have any questions or concerns.
Cheers,