Can I alternate between server and client using an ESP8266Wifi for the networking device?

That's interesting - I wonder, does it still work as a server after that error? The ESP8266 is reporting no change, which is being treated as an error - however it might not be - it might just be reporting that a server is being created with the same details as last time and the driver shouldn't be creating an error.

Does it work if you do createServer again? If I'm honest, repeated listen, close, listen isn't something I have tested, and it may be that it just doesn't work. Realistically though, using createServer again isn't actually very slow or wasteful, so it could be an idea.