In my case, the problem was that a transactional method threw a custom exception that was intended to be caught by another method:

@Transactional
public void foo() throws CustomException {
    if (...) throw new CustomException();
}

and in another class:

try {
    x.foo()
} catch (CustomException e) {
    // do something
}

However, the CustomException cannot be caught because the transaction is aborted as soon as the exception is thrown.

This article was very helpful for debugging.

Solution

The solution was to add @Transactional(noRollbackFor = CustomException.class) to the foo() method so that the exception is thrown to the caller.