Hi folks, I have to appologize for not checking my bug reports. There is no excuse for that. I am very sorry.
I have applied Moritz' patch to the source repository on Salsa and verified the GUI using Gnome on Bookworm. The error message is gone now. @Ian, if you have some spare minutes, would you mind to upload the fixed version 1.6.0-2, if it looks OK to you? There are no source code changes, just a modified configure command line and some additional dependencies. Janitor made changes to the debian directory on Salsa as well. Included. Thank you very much Harri