As a reseller, most errors in DirectAdmin's File Manager occur when you open File Explorer as a specific user, switch users in another browser tab and go back to the File Explorer tab to make any changes.
The solution is to close the File Explorer tab where this error occurred, re-enter the user account where you want to make changes, and reopen the File Explorer on that account. As long as you want to make changes via File Explorer on that account, you cannot switch to another user in the same browser.
If you want to make changes to multiple user accounts, there are several solutions. You can use the incognito function of the same browser to get an additional session. If you want more sessions, you must create and use different profiles in the same browser or other browsers.
This is not a DirectAdmin bug. This is how it works, and the management of sessions is carried out to identify which user is logged in in a given browser. The same occurs in any other web control panel (e.g., cPanel) whenever you have a reseller account and try to manage multiple user accounts in the same browser.