I have just received this error while attempting a full sync of NetSuite to Zendesk:
Error on sync: Invalid searchMore operation. Please make sure that you have had a successful search operation before you can perform any searchMore operation. (Error code: INVALID_SEARCH_MORE)
That error means there is another application doing a search while SyncApps is in the middle of getting a search result. The next sync should be fine if there is no interruption.
Do you have other integration/application connect to your NetSuite other than SyncApps? If so be sure and schedule your sync to run at another time to not interfere with other search occurring in NetSuite.