Hi all, I know this is a pretty old thread, but Esri Support recently received a similar case and I wanted to share the resolution here:
1) The error did not persist when we ran the Multiple Ring Buffer tool in a new ArcMap project.
2) To go one step further, we opened the existing ArcMap project and noticed that the Multiple Ring Buffer tool's Environment settings for Current Workspace and Scratch Workspace were incorrectly set to a nonexistent folder path.
3) To resolve the issue, we renamed the incorrect path to match a folder that exists on the user's machine. In Windows File Explorer, we obtained the correct folder path by navigating to Documents > ArcGIS > Default.gdb, and copied this path from the address bar. The folder path should look something like C:\Users\<username>\Documents\ArcGIS\Default.gdb. We then pasted this folder path into the Current Workspace and Scratch Workspace boxes in ArcMap.
4) After that, we successfully ran the Multiple Ring Buffer tool with no error messages.