This error occurs when the Sentinel software protection which Caldera uses is no longer operational.
Common reasons for this to happen is when older software like Xrite iPublish, EFI Fiery rip etc. downgrades the security software.
Using CalderaDock version 1.6.1 or newer one can reinstall the Sentinel software protection.
Similar error (same reason) could also occur when attempting to save the fingerprint from the Caldera Key Manager:
NOT Defined - Unable to get fingerprint file (00210002)
Or when trying to apply the license file/update in Key Manager:
Protection Communication Problem 00210002
More on CalderaDock:
CalderaDock product presentation
CalderaDock installation:
Manual installation, Linux (Debian)