I have the same issue, tried both firefox and chrome, also getting the following error on my website: Warning: Unknown: write failed: No space left on device (28) 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
No code has been changed so I assume it has to do with the previous downtime problems with disk set to read only?