The workround for this issue is : Close app. Login but don't open the Search Alliance (Alliance Gate) menu until 5 minutes or more. This is to load the alliance list properly.
Another workaround (Android) is clearing app cache. It should work after that.
Always wait until several minutes before clicking the Alliance Gate menu.
If you're not in a hurry, this bug fixes itself after an hour or two, so you can wait.
It's been a known bug for a while, I think I posted this before. So annoying, cause I was gonna join an alliance only to donate to my other account very quick, but I failed to join eventually