The brand new version of the 404page plugin fixes some issues and brings a lot of improvements.
A support topic pointed me at the fact, that a plugin can trigger a post query before all WordPress core files are loaded completely. This could break the WordPress admin because the 404page plugin tries to access a function that does not exist at this point if a certain core file was not loaded. This is fixed now.
Version 2.4 introduces a new way to load a translated page with Polylang. This solution now also works in Standard Mode, so it’s no longer necessary to automatically switch to Compatibility Mode if Polylang is detected. That way a another problem is solved.
The guys at WPML noticed that if the “Hide 404 page” option is activated and WPML is active only the 404 page for the main language was hidden, but not all translated 404 pages. The new vewsion also hides all translations.
Besides a problem regarding post status is fixed.