It may be serious - if the bark has previously cracked and split in various places, there's obviously something going on, particularly now that it's started oozing sap. If the sap becomes foul smelling or frothy, that's definitely slime flux - it may be better to have the tree removed or cut it down in case it becomes unstable, but you could just wait and see what happens over time if its not in danger of falling onto anything much nearby. You might seek the advice of a qualified Tree Surgeon for further advice.