JIRA ticket [LB-1113] Improve Dialog for "Error while submitting review to CritiqueBrainz" - MetaBrainz JIRA
The JIRA ticket goes into detail, with steps to reproduce fault + possible improvement ideas from yours truly.
Long and short of it, if you use the mini-review feature in LB (which is super cool btw) it tells you that your star rating is optional, but if left blank it returns a mean old non-human friendly error that your rating is out of range; thus meaning you must give your mini-review anything from a 1* to a 5* rating.