Hi Geert,
Your PixInsight installation has been messed for some reason; your TIFF format support module has been uninstalled. Do the following to reinstall all available modules:
- Run PixInsight.
- Select Process > Modules > Manage Modules from the main menu.
- On the Manage Modules dialog, click the Disable All button, then the Done button.
- Exit PixInsight.
- Run PixInsight.
Now everything should work normally.