Collectd stops after several hours

I installed & activated collectd with a few plugins. I also moved the database to a non-volatile location (USB-connected microSD). Unfortunatelly, the daemon dies after some time although the router is running fine (no reboot).

The last 2 columns illustrate the behavior:

http://pasteboard.co/kSStKOsym.png

After manual restart, collectd runs OK…for a while, until another death.

Any idea? TIA,
H.