Hard to say. Try clearing cache (/var/cache/).
Thanks. I have tried that. After checking it caused by the debug tries to serialize data to a file and the template I use (vivashop) is using closure in their template vars. I commented out the line to pass the exception but looks like the template does not support 4.3.8 yet anyway.
I gave up and reverted to 4.3.6 which is still working fine.
Cheers!