Good morning Marian,
When I tested my nintex workflow (vacation requests) as an admin your previous resolution worked. But as employees are requesting time off the workflow gets stuck in Pending with an error "The workflow could not update the item, possibly because one or more columns for the item require a different type of information"... access denied. I checked permissions and the employees have read access to the calendar and contribute access to the request list. It was working before until I made the changes for isAllDayEvent. I have searched online for some guidance but did not find what I need. I did notice others having a similar issue. Can you assist me with this? I appreciate your help in advance.