Solved it!
One problem was that the save command is not available as soon as the layers are flattened/changed, so it seems to have skipped all commands after that. Another problem I found was the open command - it was only opening a specific file and not each file in turn.
So... the procedure is:
Make a new image, blank, black background.
Select all and copy it to the clipboard.
Have all your source images in one directory and make a new directory for the target directory.
The Action will be as follows:
1. Make Layer.
2. Paste.
3. Set current layer blending mode to lighten.
4. Flatten image.
5. Select all
6. Copy
(This is one way of doubling the length of your timelapses when you don't have a lot of data.
)
Al.