Recently I started getting System.IO.PathTooLongException as soon as I type in the LINQPad editor.
Sorry about the ridiculously small image...
Have you submitted the error? Can you click on "View More Details"?
Take a look in the folder where LINQPad looks for .snippet files. By default, this is My Documents\LINQPad Snippets, unless modified in Edit | Preferences > Folders. Are there any unusual subfolders or files in there?
Also check the Visual Studio snippets folders for anything unusual (LINQPad looks in these, too):
My Documents\Visual Studio 2019\Code Snippets\
My Documents\Visual Studio 2017\Code Snippets\
My Documents\Visual Studio 2015\Code Snippets\
My Documents\Visual Studio 2012\Code Snippets\