dir function is not working on colleagues PC

Instead of using drive letter for your path... try using UNC path.

"T:\Reports\Rueckstand\"

Ex:
I have network drive z:\ (Media) mapped on my environment. But UNC path is \\ServerName\Media\Media

If you don't know the UNC path. You can use cmd.exe and run 'net use' command to find which drive letter corresponds to which UNC path.

EDIT: When using 'net use' command. Make sure to run it as specific user that is needed for. As drive mapping is specific to user context. For this reason, I used to use GPO logon script to ensure consistent mapping of network drives across all users based on role.