The problem is for tables which have columns of type longtext, not null.
When going to system info | upgrade check, the sql generated for mysql will be

column xxx type longtext not null default ''

which is not valid.

The fix: in the .dba file, replace

'type' => 'longtext',
'not_null' => '1',
'default' => '',

with

'type' => 'longtext',
'not_null' => '1',
'default' => null,