How can conflicting Excel add-ins cause this error, and how do I disable them?

Add-ins can sometimes interfere with Excel's saving process. To disable them, go to File > Options > Add-ins. At the bottom, change 'Manage' to 'COM Add-ins' and click 'Go'. Uncheck the boxes next to the add-ins you want to disable, then click 'OK'. Restart Excel and try saving again. If the error is resolved, re-enable add-ins one by one to identify the culprit.