Prob showed up again. I installed a new Plugin, everything was fine – it got activated and is running fine. Then I installed another one (WP-Polls), now again i cannot activate it, nor can i de-/activate any other plugin. Quite annoying.
I might add: There’s no Errormessage, only the “Plugin activated” message, as if everything was normal – only that the plugin remains in its old state.
Update: Login out and in again resolved the problem. Still a strange bug.