syntax highlighting doesn't work after restore a previous vim session

I had the same problem; if I saved sessions without 'options' in sessionoptions, when I reloaded Vim, the buffers were reloaded, but without syntax highlighting.

The solution is to use an autocmd with nested when reloading.

Wikia has an extensive article about loading and saving sessions. The 'nested' option is mentioned at the bottom.

I use a modified version of this StackOverflow answer, here it is:

fu! SaveSess()
  execute 'mksession! ' . getcwd() . '/.session.vim'
endfunction

fu! RestoreSess()
  if filereadable(getcwd() . '/.session.vim')
    execute 'so ' . getcwd() . '/.session.vim'
    if bufexists(1)
      for l in range(1, bufnr('$'))
        if bufwinnr(l) == -1
          exec 'sbuffer ' . l
        endif
      endfor
    endif
  endif
endfunction

autocmd VimLeave * call SaveSess()
autocmd VimEnter * nested call RestoreSess()

set sessionoptions-=options  " Don't save options