this is a known bug, reported before, which occurred with Mac OSX 10.11 first.
Another work around would be to use Send Mail script without option [_] "Do not display dialog"
Thanks for the quick response! Very helpful.
another attempt is to use script task with Option:
Send Mail [Send via SMTP Server ; No dialog]
This is considered best work around as long as it is used for any in-house solution since you have to fill-in all your private credentials for Outgoing SMTP Server (Username, Password, E-mail Address).
To put this even further: I registered a dedicated free Email-Account (gmail, Yahoo, …) for SMTP credentials with my public deployed solution.
I see, so as long as I'm not trying to use Mail, it works. This is a not an in-house solution but I had not intended to use SMTP Server directly unless and until I had to. I can set up a special email account easily enough. The whole issue then goes away from the end user's perspective and for me, it might be a permanent solution. Nice! Thanks again!
This issue has been addressed in FileMaker Pro 15.0.3.