Since you can comment using the same username and email ID and domain name as this visitor then it surely does seem like an IP block, which means it's not something b2evolution has done. B2evolution in version .9.1 doesn't have any built-in IP banning mechanisms. There are some IP banning hacks, and some of them use different external lists to decide what IPs to ban. If you've installed any of those hacks that's where you should be looking. Also, and this too is something you'd know about, since you can ban IPs at your host it's possible you've banned this visitor's IP. Wait that doesn't make sense since she can see the web - just not comment.

Ask her to try using the same 'credentials' that you tried with AND using the exact same comment you successfully posted. It is remotely possible she is trying to post a link that is getting blocked by the antispam keyword list.

EDIT: post a link in a comment is what I am refering to!

You're right: no external IP-banning, and since I couldn't find anything about IP-banning in the default installation of b2evo, I already guessed that this wasn't the cause.

I've been given a screen dump of the reaction form, right before she hit the "submit" button, and when copying this, I was able to comment. The body text was nothing more than "test test test". Which is a nice way to indicate a test, I gather ;-)

Any other ideas?

Wow. So yeah: 'test test test' should pass pass pass, especially with the same credentials that you were able to comment with. 2 more thoughts before my futility is revealed to all: Is there any chance this visitor is using www when your baseurl doesnt (or the other way around of course)? I've no idea why I think that might matter, but since I'm currently changing hosts and my new host claims leaving out the www might cause 'problems' without specifying the problems I thought I'd ask you about it. It does cause cookie issues BTW, as the cookie will be set according to the baseurl value.

Second, is there any chance your web host is blocking the IP of this particular visitor?

Oh and was this visitor ever able to comment? If the answer is 'yes' then you have an additional piece of information to try to identify the problem with. Specifically you can say "on this date it was good but on this date it was bad" which means you look for a change (at your host perhaps) between those dates.

Unrelated: my new host is an idiot. Insisting on having www in your url is like saying you have a website on the internet. WHERE ELSE WOULD THE WEB SITE BE??? http://no-www.org/

Back on track: keep us all informed about what happens with this problem. Knowing the resolution, assuming and hoping the issue gets resolved, will help countless generations of humans.

The solution turned out to be quite simple. Turning off Norton Internet Security did the trick... For some reason the reaction page of my weblog is not safe according to Norton...

Case closed ;-)