are you exporting the csv when you have “search in all messages” selected? If yes, than reduce the selected time range to for example 14, 20 or 25 days (just start with large ranges and reduce them until it works).
I have the same issue.
When the search is in 7 indices it works great but when i try to search in 8 or more indices the CSV export fails (gives CSV with only 10 lines).