@romungi-MSFT As of today, the problem no longer occurs.
Shortly after creating this question, I was unable to use the Speech Studio for about 20 minutes.
After logging in, I was shown a loading screen continuously.
After I was able to use it again, the problem no longer occurred.
The problem only occurred with this one voice. (de-DE, KatjaNeural)
And could be recreated independently of the account.
The problem even occurred on the demo page when SSML was adapted there.