Hi Rich,
I appreciate your updates and sorry for the delayed response.
Considering the Office files work fine with the Check in/out button, it means both the SharePoint online service and the Document Connection client work normally. Therefore, this issue may be caused by the SharePoint or the Document Connection compatibility with the .ai files. To find the root cause of it, could you please check to see whether the .ai file can check in smoothly on the SharePoint site?
1. Access your SharePoint site and navigate to the library with this .ai file.
2. Check to see if the .ai file can be Check In when it is in Check out mode, and vice versa.
Based on my test, it seems to work fine.
If it works properly on the online site, it may be an issue related to the client side settings or compatibility.
If the issue persists on the online site, I’ll do further research to troubleshoot it. For example, how about uploading a new .ai file to another library to check it? The performance will be helpful for further analysis.
Thanks,
Maggie Li