Can you attach your Sources/ManageServer.php file here please.
Does your host allows serialize() to be used?
Yes, I run my own server. PHP version is PHP 5.5.9-1ubuntu4.9.
Does the other settings gets saved correctly?
Yes, it seems so. I can check and uncheck options, enter integer values, and set text for email just fine. The only issue appears to be with saving group selections.