When saving an Agent Configuration with more than 1024 characters in the HTTPS Domain field, you see an invalid character count in the validation popup dialog.
DLP 15.8 GA and lower.
This occurs because the validation code reads the character count from the Off Network HTTPS Domain filter even if that option is disabled.
A fix for this issue was released in 15.8 MP1.
Use a text editor to count the characters you are placing in the HTTPS Domain filter field and ensure it's not greater than 1024.