I get this error when the timer is still going to join the mission, and sometimes once or twice immediately after the timer ends. You should be able to get in if you just try again. Please let us know if this isn’t the case.
This is not true. Pantorex does not have a 1 per week limit.
However, there is a bug where sometimes, during the first minute or two of the guild event starting, it will abruptly end. This will not end the fight against Pantorex, however killing him will provide you no rewards. This can make it very difficult to notice.
You can check to see if the guild event has ended by having someone wait outside before entering the guild mission. If Pantorex has lost 10-20% HP and that player cannot enter the guild mission, then it has ended. An ended guild mission does not allow you into the instance, and the post will also disappear if that player relogs. If this is the case, everyone needs to relog out of the mission and the guild master must restart the event. If Pantorex has lost that much HP and they can still enter, it should be fine to have them enter and defeat the boss.
This is indeed a bug and I am working to report the cause to IMC. For now, I hope this workaround allows you to deal with the issue.