Chances are good that it's an antispam thing blocking your post. To find out what part of the URL you're trying to link to, visit your antispam tab and use the filter feature to pick out pieces of the URL to see what's the problem - if any. So like if you want to link to blahblahblah.com/foofoofoo.html you can start with "blah" as a search term, then go to "foo", and see if you can nail down the part of the URL that the antispam system isn't allowing.

After that you can decide if you want to unban that particular keyword or not. If you do then you simply click the green arrow next to the keyword, and go back to posting your post.

Either that or it's something else!

Thanks for your answer. Yes, I know how to check this manually. But this might somentimes be very tricky.

What I wanted to implement is somethink like a message "Invalid URL ["Blacklist Entry which caused the message"] so that I can immediately see what blacklist entry caused the problem.

Any idea for that?

Thanks,
Michael

Ok, did find it by myself.
I implemented the changes described here http://forums.b2evolution.net/viewtopic.php?t=5291.

The location where you have to change this in 1.9.x is in the file inc/_misc/_misc.funcs.php. Search for "Invalid URL".

The code is almost the same as described in the above forum message.