In this case the customer had a program called CAX Interface (a customised interface for Mechanical Desktop 6 and SAP) that is using tcl as well, including an environment variable called tcl_library pointing to its installation. When this environment variable is deleted, then ANSYS Classic can be accessed from Interactive or Workbench. The above variable is needed to run CAX. This variable can be active without affecting ANSYS. For this CAX needs to write their code independent of setting tcl_library.