Are you getting this error for AES itself or is AES receiving the error from monitored applications/systems?

The property reference cache can be a bit tricky to tune. As a single property name can be defined in multiple classes, the engine has to figure out the appropriate class context any time you reference a property. To improve performance, engine keeps a cache of fully qualifies property name references with links to the definition. Properties are typically referenced on a given page, top level class and specific sub-class type -- so the engine uses a fully qualified reference in the cache.

The cache by default holds 100k entries. If the fully qualified clipboard path includes transaction-specific data  (like Contract.Coverage(life).premium vs Contract.Coverage(auto).premium   or address(27516).street ) the cache will fill up and engine will need to purge older / not current references to make room for new entries. 

Purging the cache is expensive. If you are continually purging, make the cache bigger.

Don't make it too big though (a) dont want to blow up JVM heap   (b) a huge cache will fill up rarely, but when it does, it will take longer / more work to purge it.

There are some system setting to change cache size and cache behavior. Feel free to engage Pega Support to provide expert advice