Well in fact - despite being flagged as an ERROR it seems it is benign enough!
When a client roams from AP1 to AP2 then AP1 will send a log message as shown below.
acsrvc_receive():88:32:9b:aa:a1:88 station of VAP c0:8a:de:3c:e0:e8, last stats reported from unmatched VAP c0:8a:de:38:86:08
This log can simply be ignored as this is not harmful.
Perhaps Ruckus should re-classify this as an INFO alert instead of ERROR