wu976544729 You are likely encountering an issue where backup saving fails due to an overly large Spine project file that existed in versions up to 3.8.99. The following thread may be helpful:
https://esotericsoftware.com/forum/d/6451-error-saving-project/15

Unfortunately, as reported by users in the thread mentioned above, there seems to be no solution other than optimizing the project in version 3.8.99. However, we have not received any reports of this issue in versions 4.0 and later. If optimizing the project reaches its limits, we recommend upgrading to version 4.0 or later.