Error on core_v2_abuseReport.Create

Use core_v2_forumThread.ContentTypeId instead of ForumReply.  The actual exception is being hidden due to a secondary exception when the Sandbox attempts to display a Report object that was created due to an exception.  Try the following, it should display the actual exception when one occurs instead of attempting to display the whole Report object.

var thread = core_v2_forumThread.Get(13);
var report = core_v2_abuseReport.Create(thread.ContentId, core_v2_forumThread.ContentTypeId, {});

if (report.Errors.Count > 0) { return report.Errors; } else { return report; }

Thank you! While we are on this topic, are there way to create custom abuse reasons?

AbuseReasonId is just an integer field, so any integer value could be used.  But it would not have much meaning in the community moderation panel.  You may be able to make use of it in a custom moderation tool though.