either of cron or loop can be set and unset, mark invalids as invalid and unfinish

server/ajax/form/cron.go

@@ -1,6 +1,7 @@
 package form
 
 import (
+	"log"
 	"time"
 
 	"github.com/robfig/cron/v3"
@@ -11,6 +12,7 @@ type Cron string
 func (c Cron) Next(since time.Time) time.Time {
 	schedule, err := cron.ParseStandard(string(c))
 	if err != nil {
+		log.Printf("failed to parse cron %q: %v", string(c), err)
 		return time.Time{}
 	}
 	return schedule.Next(since)