Many times I have faced the same issue i.e. lost the mouse control when performing the drag and drop in the virtual box. I am not sure what happened internally but with the below procedure I got the mouse control back.

List all the processes using the ps command and check if there is any match with "draganddrop"

ps -ef | grep -i dragand
mkbella+ 21155  2053  0 17:47 ?        00:00:00 /usr/bin/VBoxClient --draganddrop
mkbella+ 21156 21155  0 17:47 ?        00:00:01 /usr/bin/VBoxClient --draganddrop

Now, manually kill the above two processes using the kill command.

kill 21155
kill 21156

Killing these processes doesn't resolve the issue as we need to relaunch this process again. So type the following command and re-launch the "draganddrop" process.

/usr/bin/VBoxClient --draganddrop

You must get the mouse control back after following the above steps. If not, try it one more time.

All the above steps can also be done using the following single command.

ps -ef | grep -i dragand | grep -v grep | awk '{print $2}' xargs kill && /usr/bin/VBoxClient --draganddrop