SMF-2.0.14 PHP-5.6 can't login

[Palloy]

Updated SMF 2.0.13 to 2.0.14 OK.
I don't think there are any mods - how do I tell?
Updated from PHP 5.6 to 7.0 OK.
Basic functionality test OK.

A WordPress Blog on the same domain now gives a blank page.
Reverted to PHP 5.6, WordPress now OK.

Then I get reports of users/admins not being able to log in to SMF.
Password reset sometimes works (sometimes doesn't ?)

It is hosted on a VPS, but I can't find php_error.log or apache error.log