Default login page problem

If you deactivate the aiowps plugin from the wp admin plugins menu, then once the aiowps is inactive your site will work as if you never installed aiowps because upon deactivation this plugin will also remove any firewall rules it may have added in the .htaccess file.
If you deactivated aiowps using some other method, eg, via ftp then it is possible that the .htaccess rules from this plugin will still be there because the plugin has no way of detecting a deactivation event when it is not triggered via the standard means.

Do the following check:
when the aiowps plugin is deactivated, check your .htaccess file and if you see any firewall rules from this plugin just remove them.
Note: any rules added will be contained within the following tags
BEGIN All In One WP Security
END All In One WP Security

If you don’t see any rules inside your .htaccess file when aiowps is inactive and you still have issues logging in, then this plugin is not the cause.

The plugin was deactivated from the WordPress dashboard and the site then immediately white-screened.

when I checked there were no rules in the .htaccess file.

What about database entries in aio_wp_security_configs ?

Could they be causing this perhaps?

Is it worth deleting them to test?

What about database entries in aio_wp_security_configs ?

Could they be causing this perhaps?

No they will have zero effect and they won’t be used when the plugin is inactive.

Try checking the size the of the following tables:
aiowps_events
aiowps_failed_logins
aiowps_login_activity
aiowps_login_lockdown

These shouldn’t be huge because the plugin has an auto-pruning process.