I have been struggling with a an error in a plugin for MS Dynamics CRM Online. (see below). It appears to happen at random times, but more likely to occur when the activity is h
BlueSam was really close, it turns out it was not our plugins that were causing the issue, it was Microsoft's ActivityFeeds plugins.
After disabling the ActivityFeeds.Plugins.CaseCreate: Create of case for account and Create of case for contact, we have not gotten any more "There is no active transaction" issues.
I hope this helps someone.
I was receiving this same error in CRM online 2015.1. After much pulling of hair and gnashing of teeth, I was finally able to figure out every single plugin step for the Activity Feeds Plugin had been duplicated. I removed all duplicates and the issue resolved itself.