Mailbox with rule set, very simple for this matter = if delivered after date
& if delivered before date reply with text

Sometimes it will do, other times it will NOT do the reply

And selecting a bunch of mails in Mailbox & executing the rule by hand (Run)
with Event set to User Activated (or anything else) gives error Execution
failure (really helpful!)

Anybody has an idea how to tackle it?

What is going on?