A resource went offline for some reason (perhaps because kwallet wasn't available temporarily and it failed to retrieve a password).
The go online message box appeared upon sync in kmail, but did not set the agent online:

Agent instance ( "setAgentInstanceOnline" ) "akonadi_kolab_resource_0" has no agent interface.
"AgentManager::agentInstanceSynchronizeCollection" Agent instance "akonadi_kolab_resource_0" has no resource interface!