If your Calendar Plus Web Part not working in the Exchange On-Premises, please follow the steps below;
If you are experiencing difficulties with the Calendar Plus web Part in Exchange On-Premises, it might be due to compatibility issues with the current version. Starting from version 15.5.93, the Calendar Plus web Part is not supported in Exchange On-Premises but Exchange Online.
To resolve the issue, it is recommended to roll back to the previous version (15.5.90) that supports Exchange On-Premises.
Steps to Rollback:
1. Uninstall the current version (15.5.93)
2. Install the previous supported version (15.5.90).