Standalone convert does not produce png, while the command "convert <INFILE> <OUTFILE>" works well

[Saltsmart]

I use the standalone package to produce png from pdf. It functioned well before, but today the bulid process end up with a warning and no png is exported:

Conversion unsuccessful!
(standalone)    There might be something wrong with your
(standalone)    conversation software or the file permissions!

The MWE:

% main.tex
\documentclass[convert]{standalone}

\begin{document}
aaa
\end{document}

Run "convert main.pdf main.png" causes a warning: convert: profile 'icc': 'RGB ': RGB color space not permitted on grayscale PNG ``main.png' @ warning/png.c/MagickPNGWarningHandler/1526., but main.png is produced anyway!

I try to export a colored picture, but it is not exported by standalone as well:

\documentclass[convert]{standalone}
\usepackage[]{xcolor}

\begin{document}
\color{blue} aaa
\end{document}

I use Visual Studio Code + Latex-workshop for latex editing. Here is my environment information:

VSCode：

Version: 1.75.1 (system setup) Commit: 441438abd1ac652551dbe4d408dfcec8a499b8bf Date: 2023-02-08T21:32:34.589Z Electron: 19.1.9 Chromium: 102.0.5005.194 Node.js: 16.14.2 V8: 10.2.154.23-electron.0 OS: Windows_NT x64 10.0.19044 Sandboxed: No

LaTeX Workshop:

Version: v9.6.0