We have a working customization in place. No issues in Classic mode, but we receive this error in Lightning mode, when we add a case comment.
Our customization is to add the comment to a new field located on the case object. This Text Area field has a 255 character limit. The way it works in classic mode, you add a very long comment to the case, there is a workflow which takes the contents of the last comment, and performs a field update workflow action.  Since this text area field has a limit of 255 characters, Salesforce automatically truncates the text from the CommentsBody to fit in this field.  When I switch to Lightning mode, the same action throws the following error

New Case Comment
Review the errors on this page.

• A workflow or approval field update caused an error when saving this record. Contact your administrator to resolve it. Last Case Comment Internal: data value too large: 
• (max length=255)