I had the “white page” after upgrading from 11.x to 12.0.3.
I happened to find this page:
I followed Thomas’ recommendation: “Use APCu instead of APC in config.php. ‘memcache.local’ => ‘\OC\Memcache\APCu’, APC is no longer supported as of NC12.”
Adding the “u” fixed the problem.
Nextcloud version (eg, 10.0.2): 12.0.3
Operating system and version (eg, Ubuntu 16.04): Raspbian GNU/Linux 8 (jessie)
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.10 (Raspbian)
PHP version (eg, 5.6): 5.6.30
Is this the first time you’ve seen this error?: Yes
Can you reliably replicate it? (If so, please outline steps): Web upgrade to 12.0.3
The issue you are facing:
“White page” when trying to open admin URL.