It is possible that you have a bad outlet before the freezer. Many times contractors use the push in connectors on the back of the outlets called "stabs" (I think they should be outlawed). These knife edge connections don't do well with loads like freezers because they draw a large starting current. Start by checking the outlets close to the one that powers your freezer since you just replaced that outlet. There are usually several outlets on the same circuit the problem could originate from a working outlet that the load side stabs failed. I work my way back to the first working outlet on that breaker check it, then check the next one that is dead. It has been this almost 100% of the time. I have repaired well over a hundred over the years Only 1 or 2 had loose wires at the breaker and there one that the outlet was covered with sheet rock. If you don't find it you may need to hire an electrician that has a through the wall scanner to find breaks behind the walls with a tool like a Greenlee CS800 but these are expensive and not something a home owner would use very often.
You have the wrong breaker for what you want to do here
The Eaton DNPL (aka BRD) type of breaker is a tandem or double-stuff breaker, that has two circuit breaker mechanisms inside it that share a line terminal, but have separate load lugs. These are used extensively to "cram" circuits into a panel, as we see in your panel.
However, since both of the breakers in a double-stuff are connected to the same leg or phase of the service, you cannot use one to feed a multi-wire branch circuit (like the one you propose) without overloading the neutral, since the hot wires in a MWBC must be on different legs or phases to allow only the difference in loads to flow down the neutral, rather than the sum of the loads.
If this were a regular branch circuit, you would need to replace the bottom rightmost DNPL1515 and the empty slot with a DNPL152015, then land the wires from the old DNPL1515 on the two outer poles and the new 12AWG wires on the two inner poles. However, this creates other problems, so read on for details.
You don't have the space for a GFCI breaker
The bigger problem for what you want to do though is that you need a two pole GFCI to protect a multi-wire branch circuit, and those only come in breaker form. However, there is no such thing as a double-stuff GFCI breaker to begin with, as the electronics required to make a GFCI work take up too much space to be fit into a single breaker package along with the two breaker mechanisms.
So, you'll need to put the GFCIs somewhere else -- this could be a "spa panel" placed inline with the circuit with a 2-pole GFCI breaker in it, or a pair of GFCI receptacles or deadfronts at the "end of the line" for the MWBC in that you would have to have the two "sides" of the circuit have separate neutrals from that point on to avoid confusing the GFCIs with improperly divided neutral currents. In this situation, you'll still need to use the DNPL152015 quadplex breaker in the main panel, as described above.
Best Answer
Perhaps a neutral-to-ground short in the lamp, which is not disconnected by opening its breaker, is creating a current path impacting other circuits. I can't say exactly how as I don't know exactly how the multiple RCDs on a panel interact with one another through their common ground and neutral bus. But it's the only possible explanation. I'm guessing you are in a home with European-style RCD groups controlling everything in the house, is that right?