Check the comment from @NickD, regex:

:base-extension "css\\|js\\|php\\|jpg\\|png\\"

is indeed wrong if we follow the tutorial exactly. I removed the last \\. Still, as I said, this was working before. The actual error was that a package named cl is not supported by Emacs anymore. For that, I had to see which packages are depending on it and I evaluated following on *scract*:

(require 'loadhist)
(file-dependents (feature-file 'cl))

and saw that the package htmlize, which I use for org publishing was depending on it. The newer versions of this package are updated to work without the cl package. So, I had to reinstall htmlize basically and things started working again.

Check the comment from @NickD, regex:

:base-extension "css\\|js\\|php\\|jpg\\|png\\"

is indeed wrong if we follow the tutorial exactly. I removed the last \\. Still, as I said, this was working before. The actual error was that a package named cl is not supported by Emacs anymore. For that, I had to see which packages are depending on it and I evaluated following on *scratch*:

(require 'loadhist)
(file-dependents (feature-file 'cl))

and saw that the package htmlize, which I use for org publishing was depending on it. The newer versions of this package are updated to work without the cl package. So, I had to reinstall htmlize basically and things started working again.