Thread:Project:Support desk/Thumbnailing fails on large PNG file

I have this large PNG file in my wiki and thumbs cannot be created:

"Error creating thumbnail: Invalid thumbnail parameters"

Nothing wrong with the thumb parameters. Normal files work fine.

The problems seems to be the raw pixel ammount (4,961 × 3,508 px).

I use ImageMagick and I have set $wgMaxShellMemory to 1 GB normally and now for testing even 2 GB.

There are also no errors in PHP logs or wgDebugLogFile.

Nothing helps. Since it works here I wonder what I do wrong :/