upgrade 4.2.9 to 4.3 beta5 not working on Drupal6: 'please reset the Drupal cache' ??

http://test.naturopathica.be/civicrm/upgrade?reset=1 gives

Upgrade CiviCRM to Version 4.3.beta5

Site functioneert offline.

Sorry but we are not able to provide this at the moment.
Please reset the Drupal cache (Administer => Site Configuration => Performance => Clear cached data))

Return to home page.

Even after clearing the cache, and even after disabling caching altogether, I got the same message again and again. Clearing the cache doesn't help.
Then I tried to run a cron, and to do an update.php, which I shouldn't have done, since now in my modules list I see

CiviCRM ◦ of [12]

CiviCRM

4.3
(red cross)
Constituent Relationship Management CRM - v4.3. Allows sites to manage contacts, relationships and groups, and track contact activities, contributions, memberships and events.
Deze versie is niet compatibel met versie 6.28 van Drupal. <--- This version is not compatible with version 6.28 of Drupal
I did upload the D6 version of civicrm though !

When trying to update civicrm I got now
'page not found'

I guess I'll have to upload the old version again, update.php again, and then reload 4.3 again.
But what can I do about the 'reset cache' message ?

Upgrading to version 7 is not yet possible for us, and we need 4.3 to solve some persistent bugs.