Actually DEVONthink doesn’t apply any special permissions or hides the folder on its own. Is it viewable after renaming and/or rebooting the computer?
1 Create a folder with the same name
2 Import all contents in the faulty folder
Software is expected to be more robust
It’s actually the first report of its kind. Additional details (like full path of the folder) would be useful or whether a simple folder named “Test” works.