PR55-1A with PR52-3B - Batteries Dying

Hey guys, I have a couple of these units for counting pulses, they are wired to open collector outputs. One of them has died twice over a 2 month period, whereas the other two have been deployed for 4 months and have not budged from 3.3v. They are all configured the same. These boxes also have the external power option with barrel connectors but this is not currently used. Any ideas as to why this is happening?

I replaced the unit that was dying and noticed some corrosion on the internal side of the m12 connector. Can I purchase just the connector assembly?