MemoryCache always returns "null" after first expiration

I have the following problem, I have this kind of cache management:

public class CacheManager : ICacheManager {
 
     private readonly ObjectCache cache;
 
     public CacheManager() {
         this.cache = MemoryCache.Default;
     }
 
     public void AddItem(string key, object itemToCache, double duration) {
         var end = DateTime.Now.AddSeconds(duration);
         this.cache.Add(key, itemToCache, end);
     }
 
     public T Get<T>(string key) where T : class {
         try {
             return (T)this.cache[key];
         }
         catch (Exception ex) {
             return null;
         }
     }
 
     public void Remove(string key) {
         this.cache.Remove(key);
     }
 }
 

is quite simple.

edit

Problem: When the cache for any key expires, all the following calls to get any object in cache return "null". I check the keys and are always correct.

The problem occurs only in the client's server in my PC and my server works perfectly.

thanks.

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