Every week or so for no reason I can tell, my RB411U reboots again and again for anywhere between 10-20 minutes.

The log only shows "system error critical" and "router was rebooted without proper shutdown (cause 1)".
Any idea why it would start doing this? I haven't updated it or made any changes to the configuration.

How are you powering this unit? The "cause 1" is normally a power failure, like the power plug is pulled without using "/system shutdown".

Thanks for the reply.

It's is been powered by 12V battery + solar setup. Power is not an issue though as other parts being powered off the same equipment works fine.