After changing encoding, lyx don't regenerate every file, and so view->pdf may fail as latex uses a new encoding scheme with files that had the old encoding.
To reproduce: Put a TOC in a document. Add some non-ascii such as "æøå" so it goes into the TOC. Verify that this works with view->PDFlatex Change to some other encoding, like utf8-plain. Save, exit, restart lyx. Try view->pdflatex. This may or may not work. The view->pdflatex is only there to create temp files. Set the encoding back to something sane again, like "default". Try view->pdflatex again. Either it fails with "unknown symbols", or you get strange symbols in that TOC, symbols that just happened to be supported. Changing the encoding needs to invalidate every intermediate file that could possibly contain any text. Workaround: Make a trivial change to the document, (add one letter) then the view->pdf works again. Helge Hafting
