The issue comes when you dont have enough space for the equipment to be unmounted. I had this issue too when i mounted a battleship clok on a freighter or cruiser. You can only mount it but not unmount it.
Equipment takes up the same cargo space twice while youre unmounting it.
If what antonio told you doesnt work, the only solution is to sell it (to a POB and buy it later, which is what i did), or ask an admin for help.
"If your heart is filled with faith, then you can't fear."
as a trader of cloaks i can indeed say this is a problem. What i do is i ask people to hold a few cloaks, i unmount a specific cloak and trade the items back over and fixed, while mounting a different cloak to keep more space