Thanks @jcolonfzenpr , I hadn't thought of that!
Here is a clean log for one volume's sis job, no errors:
[sid: 0] Info (sis start vault)
[sid: 1614593546] Begin (sis start)
[sid: 1614593546] Processing transfer data logs (94314583 log entries)
[sid: 1614593546] Generating transfer change logs (82548031 log entries)
[sid: 1614593546] Sort (82548031 fp entries)
[sid: 1614593546] Dedup Pass1 (69042 dup entries)
[sid: 1614593546] Dedup Pass2 (1774 dup entries)
[sid: 1614593546] Sharing (0 return status)
[sid: 1614593546] Stats (blks gathered 0,finger prints sorted 1671058788,dups found 69042,new dups found 1774,blks deduped 0,finger prints checked 0,finger prints deleted 0)
[sid: 1614593546] End (330192124 KB)
Then, here's another from a different time with the error:
[sid: 0] Info (sis start vault)
[sid: 1614720003] Begin (sis start)
[sid: 1614720003] Processing transfer data logs (86548136 log entries)
[sid: 0] Info (sis stop vault)
[sid: 0] Info (Dedupe operation is pausing)
[sid: 1614720003] Stats (blks gathered 0,finger prints sorted 0,dups found 0,new dups found 0,blks deduped 0,finger prints checked 0,finger prints deleted 0)
[sid: 0] Error (Operation was stopped )
As you can see, no real explanation - it just pauses and then stops. The email alert gets sent when it sees "Operation was stopped". Any ideas?