Special characters in field names? (äöü, ...)



is it possible to use special characters like German chars äöüÄÜÜß in field names?
Logstash 6.5.1 is throwing:

detected ambiguous Field Reference `[Blob][Datei][L`, which we expanded to the path `[Blob, Datei, L]`; in a future release of Logstash, ambiguous Field References will not be expanded.

The targeted field name was [Blob][Datei][Länge]

Just curious if there is an option to use these chars, or if I need to avoid them.

Thanks, Andreas