Fatal Error in Memcached.getMultiByKey();

With Memcached enabled, the Vanilla Forum 2.3 works, but ocasionally I get this fatal error:
It starts working again after 10-15 minutes. But in 2 or 3 hours (I think that the problem starts when a user posts a comment or question) the error comes again.
Any ideas about the problem?
Thanks in advance!
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