On installation I turned off all languages apart from EN-GB. Upon running Texnic disabled Spellchecking because it could not find the EN-US dictionary file. I copied this in from the old version to get it working.
You seem to have CSS turned off.
Please don't fill out this field.
The described error has been added to the official bug list.
Thank you for your report.
I just looked it up. This behavior is by design. Simple reason: You had the US dict selected before and TXC tried to find it. In this case we need to inform the user that a desired dict is not available. Imagine the trouble when you would correct with the wrong dict....
Now, of course, the spell checker IS NOT DISABLED. Go to the options and select a different dict.