Cannot modify field ... Record does not exist

As I mentioned on a similarly titled thread recently, this could be caused by the record being deleted on the server. The upcoming b26 version has an extra check in this situation, when this happens it checks the list of deleted records. If the record you are attempting to lock has been deleted, it will report “record has been deleted” instead of “record does not exist”. If you still see “record does not exist” error when using b26, then there is most likely a bug that will need to be further investigated.