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SSH in browser to compute engine suddenly not working - no errors

I have a simple E2 instance running. There have been no issues with the server, but suddenly connecting through SSH in the browser (the only way I have ever connected) is no longer working. (I tried clearing browser cookies, logging in again with no success).
This loads and asks me to authorize, I confirm, but then receive a message that it failed to connect, with no error code. After this I am able to test: 1. vm status, 2. network status, 3. user permissions - which all test to okay. Looking in the logs there seems to be a request to change SSH key which I believe is normal, but am a newbie when it comes to this.
Can anyone help with the next steps - how can I regain access to my instance? Why might this not be working?

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