I am sorry if there is a misunderstanding about the capabilities of the hardware. The unit you have listed has an operational temperature of "0°-40°C (32°-104°F), Humidity: 15%-80% RH (non-condensing)".
As you have stated you are shooting in temperatures of -10°C there is a large potential for improper operation. If you have experienced an error at 1°C then this is something we can focus on, however these errors only occur at that temperature AFTER the back has been subject to unsupported Operating Temperatures we cannot ensure it will perform properly until after it has been rebooted.
The hypothesis you have purposed regarding the CF card is highly unlikely. The entire board stack is replaced when an E-Box is replaced, the unit is basically brand new with the exception of the CCD when an E-Box replacement is performed. If there were an error with the circuitury you would experience it in all other operational temperatures, not just lower temperatures. The reason we cannot guarantee proper operation below 0°C is not due to soldering or contact changes, it is simply due to the electronic signal changes at extreme temperatures. A signal delay that is expected to be a value of X due to operational temperature resistance, at an extremely cold temperature is much, much faster and therefore certain signals cannot even be detected as they are present and then gone before detection is even possible. This is not something specific to the Aptus 75, it is a basic principle of electronics.
As we designate operational temperatures with the hardware, the fact that this occurs outside of those operational temperatures (aside from the one instance mentioned at +1°C) shows that your system is working "as good as it should" as this is what we can guarantee. Other user may claim that their units operate at colder temperatures but we could not possibly base the operational quality of your unit based ont he claims of other users. There are too many variables involved in such claims, including the truth behond those claims on top of perhaps completely different workflows.
Based on the information in this case, it is not conclusive that your repair has failed in any way. If you can quantify the error that was seen at +1°C we would be happy to continue investigation, however if the errors are only present when outside of the manufacture operating temperatures I am afraid that there is no cause for concern regarding an "improperly working unit".
I apologize for any misunderstandings regarding the operational limitations of the unit and look forward to recieving more relavent information with the reported error within operational limitations.
Phase One Support