When a new object is created, the Mendix runtime will assign a unique id to this object. This unique id is not generated at the moment that the object is created. Instead, the Mendix runtime creates a cache/buffer of ids and assigns them when needed. This buffering is done by retrieving a batch of new ids from the database when the buffer is running low.
The following error indicates that the Mendix runtime was not able to retrieve a new set of identifiers for the buffer from the database:
Failed to retrieve new set of identifiers
This is usually a result of other issues, most often database connection issues. For example, perhaps the app is reaching the limit you have for the maximum number of database connections, causing queries to be slow (as they need to wait for a connection).
As a side note, for the ID reservation, we set the timeout ourselves to a default of 5000 ms. The relevant setting for that timeout is DataStorage.NewIdentifierMaxWaitTimeSo, if you'd want to, you could change that value, though it would likely be better to find out why the timeout occurs.