I have 10+ various older model RPis running already. I bought 2 new RPi 3 B+ starter packs (with original power supply!). When I insert an existing SD card it shows a low voltage warning in the top right corner and does not boot.
The SD card works fine in the older models. The out of the box SD card also works fine. I have made many customizations, so I always clone the SD cards when adding new RPi's, up to today it worked fine. Any idea why it might show a low voltage warning and how to solve it?