When I use the command 'pack external data into blend file' images imported as sequence do get packed into the blend file but when opening the blend file again the image file cannot be referenced.

You can check that the image file is correctly packed by changing the type of the image data from 'sequence' to 'file', in which case the image can be referenced and displayed.


Unpacking the packed images also does only work when first changing to file mode.

See the attached blend file for an example.

A sequence is referenced through an file input node in the node editor. Additionally the same image is referenced in the texture panel to better see what file gets referenced.

Load the file and see that no image is displayed. As soon as you change the type to file in the texture panel the image gets displayed in the texture preview. To update the node display you have to do a dummy edit in the file input node.