Hello Christopher,
i included the full stack trace in the original post i created and linked above. The server the test were run on has FINEST log levels defined for
com.ibm.form.nitro.service.level
com.ibm.form.level
com.ibm.form.nitro.level
com.ibm.form.builder.level
com.ibm.hrl.level
com.ibm.freedom.level
com.hcl.nitro.level
dleap.level
but not for: dleap.impl.service.level.
All users i tested the reminders with have a internet email in the corresponding names.nsf and receive regular emails defined in the workflow. I tested it with a recipient from a Leap Role and with writing a static email address as the recipient.
Please see the stack trace in here.
I will ask the server admins to change this log setting for a test and will update the post if there are any differences.
Additionally a colleague of mine found this support ticket which might be related: