Plug-in Error: Need to start a transaction before commit – Dynamics CRM

We have got an issue on saving the email record. PFB for complete details:

Issue background:
User selects RegardingObjectID and save the email record. On saving of email, our plug-in will fire and Associate Email TO list contacts with selected RegardingObjectID.
The email save got failed and thrown generic error message. It showed “Need to start a transaction before commit” message on downloading the error log file.
The error message didn’t provide plug-in name and any traces those available on Plug-ins which were registered on Update on email record.

Reason:
One of the plug-in has the business logic to associate Email TO contacts list with selected RegardingObjectID. If the anyone of the TO Contacts already associated with the RegardingObjectID then the plug-in will fail and throws the above error message

Workaround:
We need to check the relationship existence between those 2 records. If the relation exists skip the Association else Associate the records.