With 2.5.919 (or a prior version after applying Hotfix 7933) then the field will allow any non-alpha character except for the double quote (“)
Valid characters are alphanumeric and "_-+." (underscore hyphen plus period)
Any other character entered within the 'Secret' field will return the error:
Please enter only alphanumeric plus "_-+." characters in the "Secret" field.
If this error is seen and there do not appear to be any invalid characters, please ensure there are no leading or trailing spaces in the Secret field.
2.5.913 - 2.5.918 Install Hotfix_7933. This hotfix allows any non-alpha numeric character to be typed in the Server Secret field for Radius and Defender external authentication
Enhancement request number BFER 7933 has been added to TPAM 2.5.919. From the TPAM 2.5.919 Release Notes 'Enhancement' section:
"Allow any non-alpha numeric character, except for double quote (“), to be typed in the Server Secret field for Radius and Defender external authentication. BFER 7933"