Problem managing members in channels

I have the problem with managing the members. I have created a new channel and added a user as a member. Then he was no longer available as a member in the "old" channel. So I sent out a new invitation for the old channel. But it only arrived after hours. In the meantime I deleted the member again and sent a new invitation. Then the error message "An error occured while trying to invite the user in this channel" came up.
The user is now a member in both channels. But if I want to delete another member now, the same error message "An error occured while trying to invite the user in this channel" comes and nothing happens. The user which I want to delete, is not deleted.
Who can help me? Thanks.