When an older loco with DIP switch decoder (“mouse piano”) does not react to commands from the digital controller, there could be a simple explanation: the wires to the feeder track have been swapped.
The red wire must be connected to “B” (train current, Bahnstrom in German) and the brown wire must be connected to “0” (neutral, Nullleiter in German).
It might seem logical to connect the brown wire to “B” – but this is the wrong way!
Note when switching from CS1 to CS2 that the same plugs are used, but poles have been swapped. So unscrew the cables and swap them.
Newer MM decoders work fine when wires have been swapped. But accessory decoders normally won’t work, neither will older loco decoders.
So even if your layout was working for years, it could be that wires are swapped. But problems may show only when you buy an older loco from eBay or a flea market or swap meeting.