Seeing this pretty regularly, can only resolve by either fully reloading/refreshing the web page interface but also sometimes requires restarting the swarmui server:
11:21:03.718 [Debug] [ComfyUI-0/STDERR] Prompt executed in 5.79 seconds
11:21:03.719 [Debug] [BackendHandler] Backend request #48 finished.
11:21:05.727 [Error] [WebAPI] Error handling API request '/API/GenerateText2ImageWS' for user 'local': Internal exception: System.ArgumentException: The tasks argument contains no tasks. (Parameter 'tasks')
at System.Threading.Tasks.Task.WhenAny[TTask](ReadOnlySpan1 tasks)
at System.Threading.Tasks.Task.WhenAny[TTask](IEnumerable1 tasks)
at SwarmUI.WebAPI.T2IAPI.GenerateText2ImageWS(WebSocket socket, Session session, Int32 images, JObject rawInput) in G:_all_webuis\SwarmUI\src\WebAPI\T2IAPI.cs:line 130
at SwarmUI.WebAPI.API.HandleAsyncRequest(HttpContext context) in G:_all_webuis\SwarmUI\src\WebAPI\API.cs:line 167
Using Firefox, up to date. Happens with newly configured firefox profiles as well.