The exception (whatever it was) was caught by catch (Exception e). You didn’t log this exception, so you don’t know what it was. You should log it somehow so you know what really happened.
The problem occurs when you return -1. This allows for the possibility of inconsistent ordering, which Java’s current sorting algorithm sometimes catches. In short, returning -1 on an error means that you are asserting that both a < b and b < a are true, because the exception will be caught in both cases. This is logically incorrect. The sorting algorithm detects this and throws the IllegalArgumentException. Note that the compare method is not in your stack trace; it’s the call to Collections.sort.
In addition to logging the exception, handle it before you even get to the comparison step in your program. If you have to parse the string as an integer, do that when creating the Employee objects, so that the validation occurs before you even get to the sorting step in your program. A Comparator shouldn’t have to validate data; it should only compare the data.