LaTeX.org

A minimal working example lets me reproduce your issue on my machine, and just the issue. Please have a look at https://en.wikibooks.org/wiki/LaTeX/How_to_get_help and http://minimalbeispiel.de/mini-en.html The toc file is in the same folder as your tex file is, probably you are on a windows machine...

Open the files with notepad and copy the content to your LaTeX editor. Save the content under a new name.

Welcome to the forum. Just the error message is not that helpful in his case. A minimal working example would be best.

I guess a section entry or similar is corrupt or was corrupt. Delete the file with the ending .toc and start the compile chain again.
Can you provide a minimal working example?

Make a copy of your project on a USB stick and put that stick in the drawer of your desk. Just in case. Just to be sure. Open the different tex files one by one with a simple text editor like notepad, and copy the whole content over to your TeX editor and save it there. Once you are done and all fil...

LaTeX editors use utf-8 by default for about ten years. I suggest to make a save copy of your project and convert it to utf8.

No, they are not the same site. It is just common to leave a note if you ask the same questions in different spots. So we can check if your problem is already solved and we do not waste our time.

The symbols you found are the standard symbols and you can all scale them by hand. They are used by the physics package, which does the scaling for you.
Most advanced users prefer the manual scaling.

Yes, sure; but you would need to register the packages with the LaTeX software manually in order for them to be accessible.
I would suggest to use the package manager of your distribution.

No, this is a weak spot of LaTeX.