That error message means a mutation was above system limits. Provided that you don’t use transaction exceeding 25 entity groups, causes can include:
Exceeding 500 entities in a single commit
Exceeding 10MiB in a single transaction
Exceeding 2MiB total composite index size for an entity
Here, you can find the best practices of what to keep in mind when using Cloud Datastore. If you need further technical support, I recommend to post your code snippet of the transaction handling the batch requests to Stack Overflow using the supported Cloud tags since Google Groups are reserved for general product discussions and are not for technical questions.