It's understandable that such a basic seeming issue would negatively impact your opinion of the service, but it's also worth considering that such a basic issue

It's understandable that such a basic seeming issue would negatively impact your opinion of the service, but it's also worth considering that such a basic issue must surely be some sort of unique edge case if the vast majority of other people are claiming to be happily using the service (which implies being able to log into their accounts). Of course you don't owe Kagi anything so you don't have to reach out to support, but just something to consider before questioning someone's competence.