Trouble is with special characters in the pattern, not in the logs, right?
Then as you said, as a workaround strip down the pattern to avoid the problematic characters. Perhaps replace just that single Umlaut with a “.” (regexp special for “any character”): “verf.gbar”. So the pattern is still somewhat readable for the monitoring admin.
I haven’t tried to reproduce that issue myself, but yes, that sounds like a bug which should probably be reported.