Which one is primary - 15.7 or 15.5?
...just out of curiosity, if you already use RS for upgrade, why stopping at this early release?
ZFS may be something to think about. I've seen ZFS-based ASE 15.7/ASE16 systems moved from from 15.x that all of a sudden started to see log truncation issues. In my case there has been extensive DML activity on tables with active triggers which happen to have pretty bad impact. Hm. Curious to know what's happening in your case. I had a case opened on this - with repro shipped to TS - which led nowhere (failed to reproduce in the TS labs).
Keep us updated please.... ZFS is definitely not a good friend of ASE but it's convenient...