RobHubi wrote:What does the red stop button mean here?
The red box and green arrow indicate whether the guest was running when the snapshot was taken.
Green Arrow: The snapshot was taken with the guest in a running state (a saved-state is saved into the snapshot).
Red Box: The snapshot was taken with the guest shut down (or power-off-crashed).
RobHubi wrote:after moving the PC twice I noticed that some snapshots are missing for one VM.
Hover the mouse over the missing snapshot. A pop-up will show where the missing snapshot is supposed to be:

- missing snapshot hover.png (100.14 KiB) Viewed 1963 times
See the red arrow in the picture. This is where your snapshot is supposed to be. You need to find that file and put it back where Virtualbox is looking for it.
RobHubi wrote:after moving the PC twice
Physically moving a computer doesn't often damage the data stored on the computer unless the move was done roughly. Or unless "moving the PC" means something different than the standard definition of "to move". Can you explain what you did?
RobHubi wrote:the VM is working and I have not noticed any data loss.
This means you are using a different snapshot than the one that requires these missing disk files. Notice that in your "VMMtree" picture, under the file starting "acd7bfc4" there are two missing files and another file starting "a0feaab5". Both "a0feaab5" and the missing file starting "2384b88a" run off "acd7bfc4". From "SStree" I see that you are running from the last snapshot. If this snapshot is running from "a0feaab5", then you aren't trying to access the missing file chain, so your guest runs fine.
RobHubi wrote:I would like to clean up the error
How to do this depends in this: How badly would you howl over the vagaries of life if you lost this whole guest? The snapshots make this possibility very possible.
Snapshots make a guest more delicate and do not work as backups. They are similar to Windows' System Restore points: they are point-in-past-time markers, not extractable, useless without the base system in place, but easier to corrupt because the files are accessible on the host drive. They do not store "files", like a backup folder would. Virtualbox snapshots store changed disk sectors, which may or may not contain the entire file. They should only be used on guests you're experimenting with and with data you wouldn't mind losing. (The forums are replete with users destroying their important data because they did something wrong with a snapshot.)
Additionally, each snapshot, if used long enough, has the potential to grow to the final size of the original drive, and will not shrink once another snapshot is taken. The guest's total data size on the host could be many times what it would have been without the snapshots, and with tremendous amounts of dead data that will never be used or changed.