It's probably the same connection issue you've had before. When you first cleaned the connectors, was there any signs of corrosion on the pins or the contacts? If there was, then the protective plating on the pins will have been damaged, and they will tarnish and fail again.
If you have only owned the car for a short time, then it's possible that the previous owner has cleaned the obvious signs of corrosion already, and you are left with the hidden corrosion.
This is why it is recommended to apply dielectric grease to the connectors after cleaning, this prevents the air getting to the unprotected contacts which prevents them oxidising. Without the protection of the grease, the faults will re-occur, and you get into a cycle of having to clean the contacts so often eventually they get to a state where they have to be replaced.
Don't worry about needing a DICE. As your reader is giving you the Volvo codes, you can still install VIDA and manually look up these fault codes, this should help you get more accurate descriptions of the code and what may have caused it, and also gives you more information about the design and functioning of each of the systems in the car, which is very useful when it comes to diagnosing faults like this.
When you have intermittent faults like this, the fault codes that are left after deleting the temporary codes do not help with diagnosis. You need to check the codes that don't immediately re-occur, and possibly wait for the fault to happen again and see if any of the previous temporary codes have returned.
It's these temporary codes, that can help narrow down the area where the fault is, by looking for common factors in the somewhat random list of codes. Depending on which modules have reported communication errors (or other fault codes that could have been triggered by communication faults), you can check wiring diagrams, and see which signals are most likely to have been affected, and sometimes even which cable/connector is most likely to be causing it.
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