I recently had this error. There was lots of information on the internet, but nothing that could help me with my specific case. It would appear that a session file is not accessible anymore and this is causing the error. Aside from the error itself, I was only seeing this error on one particular browser on one particular computer and could not replicate anywhere else.
The fix I found was to remove the session file at the location defined in
open(/path/to/file, O_RDWR). To do this you need to SSH into your server. If you don’t have access to your server via SSH then you should probably submit a support ticket with your hosting company explaining which file you need removed and they’ll be able to do this for you.
Open terminal (or equivalent, I’m on a Mac) and SSH to your web server:
$ ssh email@example.com
And just remove the file. You could use
sudo rm but you’re probably better off just renaming the file incase you need it again.
$ sudo mv /path/to/file /path/to/file.backup
This should clear up the error for you. I hope this helps someone else.