This error means your site uses all of assigned memory. Default PHP value for memory limit is 128MB (very good for common websites) but your current value is 40MB (not so bad but still not so good).
This message may be caused by both using lots of plugins or having lots of visitors.
When you enable a cache plugin HMWP doesn’t be loaded on each page request so it can’t use any memory. From other side we have not a new feature in v2.11 that need more memory so I think it may be caused by visitors boost (check out you traffic log).