Timer feature unavailable, "Maximum timers reached"

Despite no timer active, the watch is unable to start one.

Steps to reproduce:
- Open menu, select "Timer"
- See "no timer active" at the top of the screen
- Try to start any timer
- Error "Maximum timers reached" appears

-> Basic timer feature unavailable

This issue first appeared on firmware version 12.74.

Rebooting doesn't resolve the problem. How to fix this?