Thanks for your reply!
With the debug mode active I get this message:
Warning: fopen(http://api.wordpress.org/plugins/update-check/1.0/) [function.fopen]: failed to open stream: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. in F:\mywebsite\wp-includes\http.php on line 984
Fatal error: Maximum execution time of 30 seconds exceeded in F:\mywebsite\wp-includes\http.php on line 984
Fatal error: Maximum execution time of 30 seconds exceeded in F:\mywebsite\wp-includes\wp-db.php on line 383
Before I didn't have to set any special permissions for the plugins folder - has this changed in 2.8.4? What permissions do I need to set for the plugins folder? I don't have direct access in this case unfortunately.