About two weeks ago I installed Antispam Bee as a possible alternative for Akismet [which I left activated after activating ASB]. Yesterday I got the first legitimate comments after having installed ASB - and both commenters were unable to post their comments at all. They got an 'Error : please type a comment'. Rest assured both of them had entered text.
After switching of ASB everything worked fine.
If any more info is needed, i.e. my settings, other plug-ins used, I gladly provide it.