Blank masthead after upgrading from 2.4.8-2 to latest 3.1.2-1

Just upgraded an old 2.4 instalation to latest.
But now when trying to configure the journal via “Settings wizzard” the masthead tab is empty.
Log says lots of:
PHP Warning: Illegal string offset 'en_US'
PHP Warning: Illegal string offset 'pt_BR'
Declaration of CustomBlockPlugin::getBlockContext() should be compatible with BlockPlugin::getBlockContext($contextId = NULL)
And finally:
Declaration of MastheadForm::fetch($request, $params = NULL) should be compatible with ContextSettingsForm::fetch($request, $template = NULL, $display = false, $params = NULL)
Malformed UTF-8 characters, possibly incorrectly encoded

Some configuration:
client_charset = utf-8 connection_charset = utf8 database_charset = utf8

Any help is much appreciated!