This behavior is due to the picture window property "Tag Prefix". Information on this is available in the WinCC Online Help "WinCC Information System -> Working with WinCC -> VBS for Creating Procedures and Actions -> VBS Reference -> Characteristics -> T -> TagPrefix Property".
The tag prefix is prefixed to all tags in the picture window object. If the tag prefix is used both for the tag connection and for the "Tag Prefix" property of the picture window, the tag name is made up of both prefixes. The tag prefix is then double in the variable name. The access error then occurs when an attempt is made to access this tag.
Entry ID 23690358 contains an example of how to use the tag prefix.