Upgrading from 1.0.15: database error during /install/

Hi,

I wanted to upgrade an old installation (started at 0.9.4, upgraded step by step over the years to 1.0.15). I …

1. copied over all the files (except index-ajax.php),
2. loaded /install/ in my web browser,
3. have chosen "Upgrade",
4. clicked through the pages,
5. fixed a small permission problem in /assets/cache folder on request of the installer
6. on the last page of the installer, it said it could not connect to the database, but offered "close" anyway.

The manager login page after that said that the upgrade processes is going on and that it may take some minutes. It didn’t I could not log in. I checked config.inc.php, but all looked fine. Reversing the upgrade and restoring the old installation, and I could log in again.

Any ideas where to search?

Regards
-Beroprac-