I have to disable filtering to succesfully cancel schedules. In the picture I’m using Offline agents only filter.
Reported this problem to kaseya support with ticket id #112935
"This is a known issue and we have reported this as bug.
I have linked this ticket to our main problem ticket for this issue and you will be updated as soon as there is further information on this. For now I will place this ticket on hold."
I believe this bug has been around since R9.1 at least ;-)