only happens when the app has many users
Sounds like you are hitting some sort of cache limit at this point. Look at whether number of SFSBs is not related to maxEntries.
But either way -- if you can, switch to AS7 (even though there might be a similar problem not yet resolved https://issues.jboss.org/browse/JBPAPP-9136).
Change the Jboss is not as easy for our app.
Instead we change the ejb and they are now Seam components, we delete the stateful anotation. This is like a workaround.