You can connect to the watch with the IDE and see the log printed in the console.
Ah, yes. Should have known this. Thanks for the reminder.
2v13.123 (c) 2021 G.Williams
Uncaught Error: Function "setBTHRMPower" not found!
at line 14 col 36 in bthrm.app.js
Error gone after updating to 13.138 Firmware.
Makes also sense since my wife has still older Firmware. So looks like some inbetween version(s) were broken, but bleading edge is ok.
Can the H10 have multiple connections? Maybe it was still connected to something else during your tries?
It can have, but that needs to be enabled via App which I didn`t do. But I made sure all other BT was turned off.