I've now discovered the cause of this problem.
The /var partition was full. It had been eaten up mostly by the contents of /var/cache/apt/ (I did apt-get dist-upgrade yesterday).
Before reporting these troubles, I had actually checked syslog, and Nagios logs very little, apart from this:
Nov 15 21:57:26 th5 nagios: Nagios 1.3 starting... (PID=14209)Therefore, I would suggest that Nagios should log an error if it can't write status.log due to disk space constraints or any other IO failure.
Regards, Daniel
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