To back up the system encrypted with BitLocker you will need to create an image from Windows
BitLocker will encrypt the data along with the file system structures, rendering the data unusable unless the right key is entered during the boot process, thus protecting valuable data.
Taking a backup with Acronis
In order to take a successful backup (image) of a BitLocker encrypted system, please use the Acronis software in Windows. Do not attempt to use the bootable media, since it will not recognize the partition, and it will initiate a sector by sector copy, which will bring issues after restoring it.
The data read during the backup from Windows will be unencrypted, since Windows will decrypt all the data on the fly.
Restoring the data
If you want to restore the entire system (bare metal restore), please boot from the media and restore all the data. Everything will work fine, with the pecularity that the data will be unencrypted. If you want to have it encrypted, please turn on BitLocker once you boot the restored system.
If you want to restore only one partition of a multi-partitioned disk, please restore it from Windows. Restoring it from the bootable media may make the restore partition undetectable for Windows.
The Cloning feature is incompatible with BitLocker. As a workaround, you can first create a full disk backup and restore it.
Acronis Startup Recovery Manager (Acronis Backup)
You can activate the Acronis Startup Recovery Manager, if you want to. The fact that the drive is encrypted with BitLocker does not prevent the activation of the Acronis Startup Recovery Manger.
Remember that the Aconis Startup Recovery Manager, just like Acronis Bootable Media, will not be able to recognize the encrypted partitions correctly, so it is not the best way to take a successful backup.
(!) If the system disk is encrypted, however, activation of Acronis Startup Recovery Manager will fail.
If you want to know more about BitLocker, please visit the following Microsoft resource.
Operations other than backup and restore may malfunction as well on an encrypted partition.