Hi! I'm guild master for magi (ign: ManaThief) and the buttons for expel/invite are greyed out. I also confirmed this is true for our alliance guild master Nakamas (VinsmoxSanji). Seems to only affect guild masters and not juniors after the latest patch.
Thank you for reporting. Seems like I needed to put the guild master at the top of the list after ordering by login date. It's likely the last person to log in (in your guild) before the server check sees the buttons, but they won't be able to use them if they are not at Jr. Master. We have a fix ready for the next maintenance but this will not be happening straight away (currently).
