An major Mendix upgrade can have effect on the memory because Mendix runtime is storing more info in the memory. 

The question is first what is just 1 scheduled event and what is it doing? If you retrieve for example all rows with a database retrieve it can happen that the data is taking more java space that you're app has available. It is better to retrieve and process in batches, instead of retrieving and processing all X rows, process this per 200 or 300 rows. Have a look at:

https://modelshare.mendix.com/models/3e805078-7b0f-478c-9ffa-0e6b3a8dcf09/batch-process-3