No Login in WebInterface after upgrade to 16.0.9

Nextcloud version : 15.0.14 update to 16.0.0/9
Operating system and version: Ubuntu 18.04
Apache or nginx version : 2.4.29
PHP version : 7.3.16

The issue you are facing:
No Login in WebInterface after upgrade
Clients are working well

Steps to replicate it:

  1. Update to 16.0.0 (or 16.0.9) with Updater (or manual)
  2. First Login: Type in User/Password
  3. Server Error shown

The output of your Nextcloud log:

{"reqId":"FKsJWULCzaH5YGSIfcRb",
"level":3,"time":"April 07, 2020 09:43:46",
"remoteAddr":"192.168.2.147",
"user":"--",
"app":"index",
"method":"POST",
"url":"\/nextcloud\/index.php\/login",
"message":{"Exception":"InvalidArgumentException",
"Message":"Username must be string",
"Code":0,
"Trace":[{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php",
"line":166,"function":"tryLogin","class":"OC\\Core\\Controller\\LoginController",
"type":"->","args":["*** sensitive parameters replaced ***"]},
{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php",
"line":99,"function":"executeController","class":"OC\\AppFramework\\Http\\Dispatcher",
"type":"->","args":[{"__class__":"OC\\Core\\Controller\\LoginController"},"tryLogin"]},
{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/App.php",
"line":126,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->",
"args":[{"__class__":"OC\\Core\\Controller\\LoginController"},"tryLogin"]},
{"file":"\/var\/www\/nextcloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php",
"line":47,
"function":"main",
"class":"OC\\AppFramework\\App",
"type":"::",
"args":["OC\\Core\\Controller\\LoginController","tryLogin",
{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},
{"_route":"core.login.tryLogin"}]},
{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler",
"type":"->",
"args":[{"_route":"core.login.tryLogin"}]},
{"file":"\/var\/www\/nextcloud\/lib\/private\/Route\/Router.php",
"line":297,"function":"call_user_func",
"args":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},
{"_route":"core.login.tryLogin"}]},
{"file":"\/var\/www\/nextcloud\/lib\/base.php",
"line":972,"function":"match","class":"OC\\Route\\Router",
"type":"->","args":["\/login"]},{"file":"\/var\/www\/nextcloud\/index.php",
"line":42,"function":"handleRequest",
"class":"OC","type":"::","args":[]}],
"File":"\/var\/www\/nextcloud\/core\/Controller\/LoginController.php",
"Line":285,"CustomMessage":"--"},
"userAgent":"Mozilla\/5.0 (X11; Ubuntu; Linux x86_64; rv:74.0) Gecko\/20100101 Firefox\/74.0","version":"16.0.9.2"}

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'instanceid' => '*******',
  'passwordsalt' => '******',
  'trusted_domains' => 
  array (
    0 => '**.**.**.**',
    1 => '<domain>',
    2 => 'www.<domain>',
  ),
  'datadirectory' => '/var/www/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '15.0.14.1',
  'dbname' => 'owncloud',
  'dbhost' => 'localhost',
  'dbtableprefix' => 'oc_',
  'dbuser' => '******',
  'dbpassword' => '******',
  'installed' => true,
  'forcessl' => true,
  'overwritewebroot' => '/nextcloud',
  'theme' => '',
  'maintenance' => false,
  'secret' => '******',
  'loglevel' => '3',
  'log_type' => 'file',
  'logfile' => '/var/www/nextcloud/nextcloud.log',
  'logdateformat' => 'F d, Y H:i:s',
  'overwrite.cli.url' => 'https://paddg.net/nextcloud',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'trashbin_retention_obligation' => 'auto',
  'appstore.experimental.enabled' => true,
  'updater.release.channel' => 'stable',
  'mysql.utf8mb4' => true,
);

PHP Modules:

apcu bz2
calendar
Core
ctype
curl
date
dom
exif
fileinfo
filter
ftp
gd
gettext
hash
iconv
igbinary
imagick
imap
intl
json
libxml
mbstring
memcache
mysqli
mysqlnd
openssl
pcntl
pcre
PDO
pdo_mysql
Phar
posix
pspell
readline
recode
redis
Reflection
session
shmop
SimpleXML
sockets
sodium
SPL
standard
sysvmsg
sysvsem
sysvshm
tidy
tokenizer
wddx
xml
xmlreader
xmlrpc
xmlwriter
xsl
Zend OPcache
zip
zlib
[Zend Modules]
Zend OPcache

Apache Modules:

 core_module (static)
 so_module (static)
 watchdog_module (static)
 http_module (static)
 log_config_module (static)
 logio_module (static)
 version_module (static)
 unixd_module (static)
 access_compat_module (shared)
 alias_module (shared)
 auth_basic_module (shared)
 authn_core_module (shared)
 authn_file_module (shared)
 authz_core_module (shared)
 authz_host_module (shared)
 authz_user_module (shared)
 autoindex_module (shared)
 deflate_module (shared)
 dir_module (shared)
 env_module (shared)
 filter_module (shared)
 headers_module (shared)
 mime_module (shared)
 mpm_prefork_module (shared)
 negotiation_module (shared)
 php7_module (shared)
 proxy_module (shared)
 proxy_html_module (shared)
 proxy_http_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 socache_shmcb_module (shared)
 ssl_module (shared)
 status_module (shared)
 xml2enc_module (shared)