The uograde seemed to go smoothly, but now there’s a blank page where my homepage should be

And when I try to access login page for the manager I get this error:
Warning: fopen() [function.fopen]: Unable to access /opt/guide/www.site.nl/HTML/nieuw/manager/media/style//login.html in /opt/guide/www.site.nl/HTML/nieuw/manager/includes/accesscontrol.inc.php on line 83
Warning: fopen(/opt/guide/www.site.nl/HTML/nieuw/manager/media/style//login.html) [function.fopen]: failed to open stream: No such file or directory in /opt/guide/www.site.nl/HTML/nieuw/manager/includes/accesscontrol.inc.php on line 83
Warning: filesize() [function.filesize]: Stat failed for /opt/guide/www.site.nl/HTML/nieuw/manager/media/style//login.html (errno=2 - No such file or directory) in /opt/guide/www.site.nl/HTML/nieuw/manager/includes/accesscontrol.inc.php on line 84
Warning: fread(): supplied argument is not a valid stream resource in /opt/guide/www.site.nl/HTML/nieuw/manager/includes/accesscontrol.inc.php on line 84
Warning: fclose(): supplied argument is not a valid stream resource in /opt/guide/www.site.nl/HTML/nieuw/manager/includes/accesscontrol.inc.php on line 85
It looks like something’s going wrong when the manager style is selected... look at the double slashes in the first rule.
Any ideas? Please?
I used PaulGregory’s instructions for upgrading as described here:
http://modxcms.com/forums/index.php/topic,8587.0.html
When I change the names of manager-old etc back to their old names, the site seems ok again - but the manager looks all weird (no stylesheet I guess). And the top menu frame is gone.
Edit: this is not yet a production site, so it’s not such a big deal. I think I’ll do a clean install for this one. However, if this *had* been a live site, I would be in very serious trouble now. So I think this issue is worthy of some attention... It might happen to others.