The error 40300 can occur for a number of reasons, due to being the default for 403 (forbidden) actions. Below are examples of known causes and solutions for this error:
Mismatched version for existing connection
This can potentially occur if you are using multiple versions of Ably within the same application. Make sure to use the same version of Ably throughout your application.
This error code usually indicates that the app you are accessing belongs to an account that has been manually disabled by Ably.
Alternatively, this error code may also appear for if you are accessing an app that belongs to a dedicated cluster, via an incorrect URL. Our Enterprise customers who have chosen to have their own private cluster are provided with custom environment endpoints. If a request arrives at the default global endpoints, the account the app belongs to in the global scope may be disabled, and as such, this error message may be shown to users. We recommend you check your custom environment settings for all connecting clients if this applies.
If you get a cause for this error not documented here, please get in touch so we can help.