I have experienced what 3rdi4sight is talking about and I have found that a workaround is to wait until a minute or more has passed before hitting the UPDATE button.
You get this message when start time and end time is the same, i.e. if you try to update within first minute of creating a new time entry.
Another workaround if you want to do the update immediately is to set the start time back 1 minute.
I reckon this issue should probably be filed under bugs rather than feature requests.