Solar Power Manager 2.0 (12v Lead Acid) - suddenly stopped, seems to short solar input

Hi all - long time reader, first time poster.

I have had a 12v solar power manager (lead acid) 2.0 (https://www.dfrobot.com/product-1795.html) running out in the field for some months now powering a RPi-zero. 

Everything was working beautifully. 

I was even logging current and voltage.  Voltage never rose above 13.5 (maybe 13.6 very briefly), and charging current rarely exceeded 200ma. My understanding is that it's well within spec.

After about 4 months the RPi went offline. 

I measured the solar input and it was being pulled down to 0.7v when connected to the solar power board. Open circuit it was producing 19v (sunny day).

The lead acid battery seems fine, sitting at 12.9v.

I took the board home and plugged it into my lab supply. It seems to immediately short the input which pulls the voltage down to 0.7 and the lab supply goes into current limiting mode.

Where to begin? I'd be happy to buy another board but this is in a really remote location and I'm concerned it may happen again - it's for remote monitoring on a farm which is a few hours drive so I'd really like to get to the bottom of it.

Would it be repairable? I'm no expert on electronics, hence the post, but if it was a component I'd be able to swap it over.

There are no physical signs of damage and it was well and truly waterproof. It was around 25deg-C the day it stopped. Some weeks earlier the day temp peaked at 41deg-C and that seemed to be no issue.

In this chart: red is current (negative = charging), blue is voltage. The last reading before the RPi went offline was 13.5v & 69ma charging.

Thanks in advance.