So I guess I tracked it down. phew
It seems to be a problem in ImageMagick that was fixed in March, 2024:
If you look at the delegates.xml.in configuration or the compiled in defaults in delegate.c you will notice the output redirection at the end.
Should %s be a NULL pointer, a file with the name (null) in the current working directory will be created. Should the pointer by accident be valid, whatever it points to will be interpreted as NULL terminated string and used as the file name.
That does explain all the random garbage files.
Now, if the invocation of inkscape was fixed in ImageMagick why does the problem still show up? In our particular case while the compiled in default is clearly correct (we run current versions of all “middleware” software in our hosting environment), most of our Nextcloud installation that are a bit older have outdated configuration files including delegates.xml overriding the compiled in default and provoking the error.
So to everyone else facing this problem: check your ImageMagick version and if that is a current one then check your delegates.xml for outdated entries for SVG/inkscape.
Kind regards,
Patrick