Hi, this morning everything worked properly without any changes. So this indeed seems to be related to something on Apple's side, perhaps with the activation of the new watch I've tested the shortcut on (it was unboxed two hours prior). It would be nice to have a proper answer from Apple about what has changed since the watch was working and was paired properly for over 6 hours until I went to bed and I stopped trying to use the shortcut.

Hi @DTS Engineer thank you for your reply but with all honestly, I don't think that the Apple Support Community has any value in this scenario. Any reply I've ever seen there are referring to basic troubleshooting guides or steps. I'd like to keep this thread open if you don't mind, with the hope that someone from Apple is able to tell exactly why that error is returned exclusively in WatchOS or another developer to say that they have already seen this error in the past and it's due to xyz. If you search on a search engine for the exact error ("Cannot send with an inactive account"), it seems to be returned since IOS 9 or above, and there are less than 10 threads about it. Regards