Thanks @joseph.
This is weird because whatever the timing of the Cloudron Dyn DNS update, the pattern is always the same and only the dashboard (my.) record is failing. The other (app) domain record updates are successful and just fine.
is it because it is the first one in the list of the batch to be updated?
Is it because there is a en issue with the content of the record ("my")?
I have no idea.
I am attempting to contact deSEC to see if I can get more info from their end and will revert back/if when I do.