There was a problem where the error message on ACIE was not mentioning the blocked Posting Date but for NAV 2018.
Since the function in BC is different, you can change the code here:
Code unit 5700 User Setup Management
[External] CheckAllowedPostingDate(PostingDate : Date)
IF NOT IsPostingDateValid(PostingDate) THEN
// ERROR(PostingDateRangeErr); //Remove
ERROR(InformativePostingDateRangeErr, PostingDate); //Add