Lua_shared_dict: no memory

Hi, we encountered a problem in our kong nodes (2) regarding this issue and this one
Kong version 0.11.2.

Each node it’s a docker container running on an EC2 instance (with 8G of Ram) the memory consumption for container is 900 Mb average with no spikes (docker stats and AWS metrics check), so basically it never hits the mem_cache_size limit.

Our enabled plugins are oauth2, cors, tcp-log, request-size-limiting, acl , and cache size configuration is mem_cache_size=2048m and DB cache ttl is db_cache_ttl=604800 (1 week).

We restarted both nodes 4 days ago and this morning they started to hit this error:
do_authentication(): failed to get from node cache: could not write to lua_shared_dict: no memory causing status code 500.

Do you have any ideas to solve this problem? Did we miss something in our configuration causing this problem?

Thanks in advance.

Moreover our complete configuration