A "repeat order" is a standing order. When you edit a repeat item, the app submits the change with the timestamp it last knew about (OrderLastModified / LastModified).
The server compares that against the current DB value. If the server's copy has been modified more recently (by the dairy/portal, another device, or a refresh job), your submission is considered stale and is rejected.
Likely root cause
The server received a change for an order whose OrderLastModified on the server is newer than the copy the app was working from — i.e., the standing order was edited elsewhere (dairy back-office, the customer portal, another device/session) after the app last synced.
A background refresh pushed updated order data down while the user had a change in progress, so the app's cached timestamp went stale.
If the user logs out of the application and then logs back in, this will force the application to refresh, which should stop the issue from occurring, as well as give some time for any other actions on the account to complete.