OK, that may help narrow this down.
If you open the tentacle manager and check the Thumbprint section, there should be some text stating what server and thumbprint it polls.
Then, in the Octopus UI if you navigate to Configuration > Thumbprint does the thumbprint listed there match what is in the tentacle manager?
If these do not match, then the polling tentacle instance will need to be reconfigured. The easiest way would be to use the tentacle manager to delete the tentacle instance and then re-create a new polling instance.