There is no error message. The issue can be observed as follows:
When viewing the MS Teams conversation the reactions are visible:

When viewing the comment in Arctera eDiscovery the reactions are not visible:

The issue is due to a limitation in the Export API's within Microsoft Graph. Merge 1 uses the Export API to archive the Teams data. As the Export API does not support 'reactions' Merge 1 can not archive them. Please refer to the following Microsoft documentation for more information:
Arctera has acknowledged that the above-mentioned issue is present in the version(s) of the product(s) referenced in this article.
There are currently no plans to address this issue by way of a cumulative hotfix or service pack in the current or previous versions of the software at the present time. This issue may be resolved in a future major revision of the software at a later time, but is not scheduled for any release. If you feel this issue has a direct business impact for you and your continued use of the product, please contact your Arctera Sales representative or the Arctera Sales group to discuss these concerns.
Please be sure to refer back to this document periodically as any changes to the status of the issue will be reflected here.
When viewing Microsoft Teams conversations in Arctera eDiscovery any reactions to original comments are not displayed.
JIRA: EVC-11318