I can think of three ways that the humans might be able to communicate with each other without the superior AI catching on to it.
Poetry
As mentioned in the question, an artificial intelligence, though it can mimic the analysis or prose and artful devices, only a human can really appreciate the ideas and concepts captured by a poem. Note that the use of a poem would not reveal messages in plain sight but use literary devices to convey the idea rather than exact phrasing.
Advantages
This seems like a better way for humans to communicate their feelings about situations and their love for each other (maybe?). This also is easy to learn and pickup, and even with the AI's massive database of poetry, even with the ability to add new intercepted poems, it would not become easy to break.
Disadvantages
Communication has a lot to do with clarity and poems may not be the best pick for this, as different people can interpret the same poem with different results.
Code with Virus
Perhaps the only weakness to this AI is anything written/spoken must be passed through some interpreter/compiler, and this means the AI could be hacked. 634
As part of the developmental process of the AI, rather than "escaping the code" to prevent a virus from running, the AI was developed to "skip over" code that it recognizes contains a virus.
When the humans discover this vulnerability, they develop loads of viruses and attempt to shut down the AI. Now, however, the remnants of the viruses they learned and tried to use against the AI, they use as the header of footer of messages with each other.
Advantages
This completely prevents the AI from reading and learning anything from a human's message.
The human's can communicate their message directly, as they know where to look for the actual message and ignore the header and footer code that is the virus.
Disadvantages
Humans have to remember and retain the code that is a virus and write it perfectly. This could also mean it takes longer to "encode" their message.
Illogical Statements
In the second <PERSON> movie (featuring <PERSON>), <PERSON> and his brother communicate with a simple code that makes complete nonsense to an outsider. They flipped any all truthiness of statements (like: "I love you" becomes "I hate you").
The humans have found that using this coupled with using sentences like, "this statement is false," utterly confuse the AI, resulting in the AI ignoring/failing to process their message.
Advantages
Once learned, the humans could get in a habit of this and learn to communicate quite easily.
Disadvantages
The learning curve might be harder than I imagine. 634