Strange indeed... I deactivated the plugin and deleted it via the Plugins section. Then I tried re-installing it via Add New > Search but with the same result. Again via the Add New > Upload... So back to good old FTP and all is well :)
I also tried the plugin on a fresh WPMU testing installation and there it did not give me any troubles at all. So somewhere there is something going wrong. A conflict with another plugin? Old settings in the database? No idea. If this is going to occur again with other plugins I'll have to take a closer look.
Anyway, thanks for responding!