Could not create alert condition - too many entities

I could not create a new alert condition. On “create” I always get the error message “We couldn’t save this condition because it covers too many entities. Please narrow down the number of entities above and try again.”
I have applied a filter ( “Aws - Rds - Db Cluster Identifier” contains “prod”) and the graph shows only three entities. Is that still too many? Or did I anything else wrong?
The alert I want to create:
Type = Integrations
DataSource = DatastoreSample
Filter = “Aws - Rds - Db Cluster Identifier” contains “prod” → Selects three entities
Thresholds = “Aws - Rds - Write Iops” above 6000 for 5 minutes

Thanks for advice

I tired it again today and now it works. I did exactly the same. Very strange behaviour.