Hey there,
thanks for going back with this info.
It seems that the problem appears to be from a time, datetime or date field. I’m not quite sure why this happens (as I was not able to reproduce it) but I could at least fix the fatal error.
Maybe you can update to version 2.28.3 of SNIP which should be available later this day. Here is how you can update it if you don’t see the update yet: https://rich-snippets.io/automatic-updates/#updates_not_appearing
Please test it again and check if the field is printed out in your schema when filled.
Greetings
Florian