Although you use the same username and password, it doesn't work all the time as your security ID no longer matches the security ID of the actual owner of the folder. There're some factors which might have caused this but the thing right now is to take the ownership of the new folder,for that you can try these steps.
1) Go to My Computer > Tools > Folder Options > View, and under Advanced Settings unmark Use Simple file sharing ( Recommended)
2) Right click on the folder which you couldn't open and go to Properties > Security ( click ok if some stupid security message comes up ) > Advanced > Owner
3) From the name list chose your name or Administrator and click to select the "Replace owner on subcontainers and object " thingie. You should get some error/ warning messages keep on clicking yes :)
4) Hope it works . Don't forget to redo what you have undone in folder options .