Your node is not located at {opt.addr}:{opt.port}. Go to the Settings Page to configure the connection.
You edited your settings and saved them but didn't reload the page
Your IPFS node doesn't allow requests from the domain you're running the app from (CORS issue). See here for the fix.
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You're downloading the app via `https`: at the moment, connecting to IPFS only works if you donwload the app via plain HTTP. If you're using `ipfs.io` please consider accessing the app via a local gateway like `localhost:8080`
Some other networking or browser security issue is preventing the App from talking to your node.