Autoexplore is known to cause this as well.
This is usually the case. Boats seem particularly prone, so check them first. Stuck units will generally remain outlined in yellow in the unit tree on the left side of the screen when you try to run the turn.
You have a memory leak problem at that point.
AFAIK stuck turns are not known to have any connection with memory issues. The bulk are due to autoexplore/pathing.