If Windows user name contains a German umlaut (e.g. “ü” in “Günter”), you can add an unknown word (via context menu “Zum Wörterbuch hinzufügen”) to the file “all.dic”, but the added word will still be marked as unknown the next time you type it.
If you add the unknown word again, “all.dic” contains the new word twice but it is still not recognized, if you type it once more.
The same installation works as expected, if Windows username does not contain a German umlaut.
OS version: Windows 11
App version: ONLYOFFICE Desktop Editors 8.2.1.38 (x64 msi)
Downloaded from: ONLYOFFICE website
Bug only, if Windows username contains a German umlaut
Example: C:\Users\Günter\AppData\Local\ONLYOFFICE\DesktopEditors\data\dictionaries\all