I have just started getting the same issue with Nitroflare, after never seeing this problem with them before and after having downloaded a file successfully in the alast day without even a hint of any issues.
I tried the method used by raidrunner (the checkbox "Use API for premium accounts..." was blank, so I selected it) but to no avail.
When I followed the dialog prompt "Enter Captcha for nitroflare.com to continue" a JDownloader windo opened which stated: To solve this kind of captcha, you have to use either the Android app via 'my.jdownloader', or our My.JDownloader browser extension. (Please note: a My.JDownloader is NOT required!)
From this window I clicked on the link
https://my.jdownloader.org/apps which opened in Chrome browser, and -- without having a clue why I was here -- I chose "Browser web interface" and eventually finished up inexplicably having to authenticate my perfectly valid NitroFlare account.
I ended up not have gotten anywhere, with my NitroFlare account still disabled in JDownloader2.
Any ideas? It's 10 minutes to 3 AM here in Melbourne, so I hope to catch up with this later today after some shuteye!