Batch classes work on a large amount of data and thus they create large debug logs.
We can have only 2Mb of debug logs for each batch iteration, thus you have to modify your code to remove extra debugs or change the debug trace flags to DEBUG only to stop unwanted hug debugs logs being generated.
There are ways via which you can find the exact root cause.
- Calling Batch's execute method directly with the set of records which are causing the issue.
- Writing proper assert based test classes and checking which lines were missed to cover and what caused the issue.