Symptom

Starting PTV Visum/PTV Vissim fails with an error message:
"The buffer file for license data has an old format"

Resolution

1) Open a windows explorer window.
2) Remove this file (for example for Vissim 2021):
%AppData%\PTV Vision\PTV Vissim 2021\licenses.data
3) Remove this file if present (for example for Vissim 2021, this file is only created if the license has been selected by an administrator):
C:\ProgramData\PTV Vision\PTV Vissim 2021\licenses.data
4) Start PTV Visum/PTV Vissim and select the license once.