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Do NOT upgrade your local Melodic installs for now [regression]

Latest Melodic sync (update of available packages) brought genpy version 0.6.15 which was discovered to have an incompatibility with custom messages built by previous versions of genpy. If you have such messages, you won’t be able to subscribe to topics or call services!

Workaround 1: update Melodic and completely rebuild your workspace (so that all your messages are built by genpy 0.6.15)
Workaround 2: do not update now, wait until the issue is resolved.

The reported issue: DeserializationError cannot deserialize: unknown error handler name 'rosmsg' · Issue #138 · ros/genpy · GitHub .

From scratch CI builds should be okay (as they will build all messages with the same version). That’s probably also why CI did not catch this bug.