Umlauts are wrongly exported if file encoding is ANSI (Notepad++ 7.8)
Exporting a text from an ANSI encoded file, that contains German umlauts convertes them into random symbols. It also combines the next character into it. This issue does not appear with UTF-8 and UTF-8-BOM files.
See also the attached screenshot.

@dfs- Which version of N++ and plugin do you use on which windows version? 32 or 64 bit.
I‘m sorry. It‘s a 64-bit Windows 10 running Notepad++ 7.8 32 bit and the latest NppExport build (0.2.9)
@dfs- Which export do you use? HTML or RTF. Or do you use "copy RTF|HTML to clipboard"? Export to HTML seems to be ok for me.
„Export all formats to clipboard“. I never bothered checking the other methods for mails. :)
Update: Does not happen when using the "Copy RTF to clipboard" command, but does happen when using the context menu entry (Plugin commands > Copy Text with Syntax Highlighting).