Horst,
We couldn't reproduce the problem using the latest public build and some test Encase images. But getting your log would still be good if possible.
Can you confirm that you added the image file via the option in the screenshot
here, circled in blue?
That is, you have selected the "Image file" radio button for the "Data source" and NOT via the "Drive letter" or the "Mount image" underlined option (which launches OSFMount).
Please make sure to add a device using the "Image file" option, and NOT OSFMount, and try again.
If you did index from a device that was mounted via OSFMount, you need to make sure that the image file is still mounted before indexing. That is, if you have rebooted the machine since mounting the image, then it would no longer be mounted and the error you see could happen.