When you force a specific culture from user.config it doesn't matter what the browser has configured in the browser, the thread will execute as the forced culture. As you can see the log entry includes the culture en-GB that the thread is executing as at the time of the error.
However, if the user is manually entering dates in en-US format then it will not parse correctly as en-GB so it will cause an error. I don't really know a good solution other than that all the editors use en-GB format when entering dates. The date picker should format it correctly but if the user manually changes it, that would cause an error.