Sorry, looking at some of the other topics discussing similar as far as I can tell, this is not solved yet.
As of a few minutes ago I am still getting different Captcha styles between Jdownloader2 and the web site!
When using Jdownloader2, I get this type of captcha
totally either unreadable or wrong type than used from their web site apparently even when typed in properly!
Notice the same File name too!
Some Nitroflare links that I have this problem with:
OS X 10.6.8 and JD2 with latest update.