Hello world!
Using Greenbone Security Assistant version 6.0.1 web GUI and trying to create under Configuration > Credentials a new entry fails with return error message:

Internal error: create_lsc_credential_omp:4506 (GSA 6.0.1)
An internal error occurred while creating a new credential. It is unclear whether the credential has been created or not. Diagnostics: Failure to receive response from manager daemon.

Notice that this is a brand new OpenVAS installation, no other credentials exist and is under Kali Linux 1.1.0a distribution.
Furthermore, he web form for NEW CREDENTIAL has fields Name, Login and Password which are mandatory to be filled.
If only Name is not null and form is submitted using Create Credential button, the error screen of ""Results of last operation" with Status message "At least one entered value contains invalid characters or exceeds a size limit. You may use the Back button of your browser to adjust the entered values. If in doubt, the online help of the respective section will lead you to the appropriate help page." appears as expected. The same is true if any other single mandatory field is given. Again as expected.
But, if Name and any other mandatory field is filled then this internal error appears.
This should help.

Dennis.