errors - Memcache server and MODX

Hi,

I have added a memcache server in aws. I have install memcache extension in webserver.

Then I change the setting in modx backend.

"cache_handler" system setting to "cache.xPDOMemCache"
add a new key "resource_memcached_server" and value = xxx.com:11211

And save.

There are some errors in log -

[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 1.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 8.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 1.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 8.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 1.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 8.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 1.
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache source data for 8.
[2015-01-23 14:17:13] (ERROR @ /index.php) Error caching lexicon topic lexicon/en/wayfinder/properties
[2015-01-23 14:17:13] (ERROR @ /index.php) Error caching lexicon topic lexicon/en/wayfinder/properties
[2015-01-23 14:17:13] (ERROR @ /index.php) Could not cache context settings for web.
[2015-01-23 14:17:14] (ERROR @ /index.php) Could not cache context settings for web.
[2015-01-23 14:17:16] (ERROR @ /index.php) Could not cache context settings for web.
[2015-01-23 14:17:16] (ERROR @ /index.php) Could not cache context settings for web.
[2015-01-23 14:17:16] (ERROR @ /index.php) Could not cache context settings for web.
[2015-01-23 14:17:17] (ERROR @ /index.php) Could not cache context settings for web.
[2015-01-23 14:17:18] (ERROR @ /index.php) Could not cache context settings for web.

What is the problem with my setting? Did I miss something?

Thanks

Keith