Apple Logo flashes on and off

My Apple Watch Series 3 GPS 42mm had low battery in the evening so I let it charge overnight, woke up the next morning and saw the apple logo flashing on and off. After following the guide (used another charging cable and plug, disconnected the watch from the iPhone …) I went to the Apple Store. They are not able to make any diagnostic and told me they can change the watch for ~180€ to a new Series 3. So I thought there could be two options: either the battery needs a replacement or there is a software bug. Since I am also not able to run a diagnostics (or to fix the software bug) I bought the iFixit kit to replace the battery. The replacement and the guides (this and that) are great. The problem remains… Anyone any idea?