This error happens when a boolean value is accidently empty (and not true or false). This happens in 'Done?' You can correct this value beforehand.
But that is not the cause of this problem.
Check the apply entity access. (should be off for anymous access) or assign the guest user write-access to the attribute.
EDIT: the anonymous user has a role in the system (security/anonymous users). You can give this role rights to run the microflow and see/edit the attributes.
Thanks for your response Chris. but I do not understand how this will solve my problem. I do not think there is something like guest access for this java action, is there? And entity access is set to false.
The attribute Done is a result of the Java action SendEmail, which comes from the module EmailTemplate. Should I change something in the java action to make this work?
The microflows triggering the action do have anonymous access, so I cant logically explain why this is not working, but I am also not a Java programmer so maybe its just something small in the Java action.