Hi there.
I had the same situation a couple times. I don't thinks something needs to be fixed, but I do think it might be just a frontend low priority bug.
I created 5 mobile supply units on Country1. Moved 3 them to Space Center and use a Cargo Shuttle to moved to another country Space Center.
What I noticed, is that Country1 will keep showing that you have 5 mobile supply units, but only 2 are in Country1. The other 3 are already at the Space Center of Country2.
So, when you try to move the 5 Mobile Units that are shown in Country1 Space Center, you will get that message.
I mean, list of mobile units are not being correctly updated on time everywhere, but the mobile unit should still be available to use through the "Mobile Supply Units" page.
Hope it helps.
Regards