Which version of RHQ? Also, was the agent actually still alive or only a zombie (I'm not sure which behaviour you describe with the "no error logs around ..") ? Was the agent printing any log lines at all after the last availability time?
Sorry for the late answer.
We are using RHQ 4.9, the agent was still printing logs, but we could not find any relevant error log.
In the meantime we set up another servers running RHQ. On this servers (different hardware config) the failure did not occur so far.
So we are investigating what could be the difference, if you have any hint where to look at, it would be great.