I've had a SoundTouch 20 for years. My wife got me a new one and it worked fine at first, but got an update and has not stopped rebooting itself since. It will turn on and show the load bar, then load up to near completion then it will black out for 5-10 seconds. After that it will show a fully loaded bar, then it blacks out and immediately starts the process over again.
We checked the back and bottom of the speakers and there is no SN on the bottom or back.
Thanks for reaching out, I am very sorry to hear that your SoundTouch 20 appears to keep rebooting.
The serial number is located on the back bottom corner of the system. You will see a QR code, around this will state your serial number. I've issued an example image below, the first digit will be 0 and the final two will be AE.
Let's attempt to reboot your speaker. Please remove from power for 30 seconds then reconnect. Do let us know if this persists after the reboot.