from my experience this happens if
a. connection is bad (bad contact or long wires)
b. spikes from mains coming in ; if you supply target not from USB (where stlink is also) your connection is prone to catch inductive or capacitive coupled spikes from mains;
i had exactly this behavior here at my PC . "connection lost" every some minutes or an hour.
then plugged in mains -> target supply at the PCs (filtered) master-slave socket strip and cable routed near usb-cable to target; target has mains filter and mains earth also; now debug can run many hours without any problem.(and still 2,5m away from PC , stlinkV3 -target cable about 20cm long, at full speed 24MHz + 16Mhz SWO)
try this.
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