iRule Editor - error when saving bigip.conf

We've just upgraded one of our LTMs to v11, but I'm getting an error when I try to save bigip.conf from the iRule Editor. Here's the error: Unhandled exception has occurred in your application. If you click Continue. the application will ignore this error and attempt to continue. If you click Quit. the application will close immediately. Exception caught in System::um:iControl:System/ConfigSync::upload_file() Exception: Common::Operation Failed primary_error_code : 16908289 (0x01020001) secondary_error_code : 0 error_string : Error opening file for write operations.