I had this issue, and long with it, I had the following text appear at the bottom of all my blog pages:
Warning: Unknown: open(/tmp/sess_dbf497a416add3796ab73cc7663d8b93, O_RDWR) failed: Permission denied (13) in Unknown on line 0
Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0
I emailed my hosting provider about it, and they fixed it on their end, but didn’t give me any details about what the problem is.
So I’d recommend you all contacting your host to see if there’s an issue on their side they can fix.