Html session file error

I'd guess it refers to PHP sessions, which be default uses file for each session. Thus no special "session for writing".

Would this error message appear when somebody tried to edit a past session file ( ie reopening a previous session)? Or is it saying that it cannot open a new session file?

I wish English-speaking developers would write either "the" or "a" instead of saving a small amount of space and making everybody else guess the meaning!

Again, this just a guess, but it just says that the file cannot be written whether it exists or not. This is usually system configuration error.

 Done - documented.