I use timers on my Apple Watch pretty regularly. Timers are good, timers are simple, but somehow WatchOS 26 has messed them up.
I have a simple shortcut, that will start a timer that ends at a specified time. Kinda like a self deleting timer, that gives you a countdown. This has worked perfectly, every single time. Until after I updated to 26. Now I occasionally get this error.

This isn’t an error for something stupid like setting a timer for a negative time. I have a check for the in the shortcut. This is just shortcuts, not being able to start the timer, at all.
The only way I have found to fix this, is to manually start a timer, and let it expire. If you cancel the timer before it goes off, shortcuts still throws this error. And no, rebooting doesn’t help.
Apple, what happened. You used to be really good at this sort of thing.