Write SQL query to solve given problem: What was the issue that the client with the longest server time faced?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T2.Issue FROM callcenterlogs AS T1 INNER JOIN events AS T2 ON T1.`Complaint ID` = T2.`Complaint ID` WHERE T1.ser_time = ( SELECT MAX(ser_time) FROM callcenterlogs )