You might want to check the versions on those that do and don't work. A potentially relevant change is
https://github.com/koreader/koreader...842fc1d2605068 which changed the behavior regarding usernames with one or more spaces at the end. They used to be allowed, and now they're not anymore.
Quote:
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Is my next step to open an issue with Koreader GitHub?
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That depends which sync server you're using.
In any case, if you do, please include verbose debug logs.