Anything listed in your error.logs. Script timing out, Maximum number of input variables exceeded, etc.?

What charset are you using?
Are you using any special encoding?

Replying back to your PM. I really dont have much time for Skype Support so I thought I will still try reply and help out here.

I have no Idea what is causing it. There has been some very good replies and the Guys replying knows way more than I do. But I will still try...

I followed the help topc and know that you have been Disabling your Products/Plugins using your Config File like "Ozzy47" Explained. I sugggest you try Disable your Products one by one Manually and then see if the issue still appears.

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First recommendation / Disable Your Products One at a Time:
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- Goto your Products Page, and Disable each of your Products one by one. And Everytime you Disable a Product then go back to the "Add Forums" page and see if the Issue still appears.

Do the same thing with all your Installed Products, Disable them one by one and then Check.

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Another Recommendation / Re-upload Fresh Set of vBulletin Forum Files
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- If you still have the same issue after having your Products Disabled one by one, Then this is the Last option I would have done. It could be that one of vBulletins Styles or one of the XML Files are currupt or maybe not fully uploaded somehow.

You can try upload a new set of vBulletin 4.2.2 files on top of your Old Files to see if that fixes the issues. If you want to do so, then Follow the instructions below.

1) First make sure to Backup your Forum Files and your vB Database. Just inscase somethign goes wrong.

2) Download the LATEST Patched vBulletin Forum Software from vBulletin.com's Members Area.

3) Beside the install folder, Re-upload all vBulletin 4.2.2 files from the "upload" folder and overwrite your old vBulletin Boards files.

- Once again, Do NOT Upload the "install" folder.
- Try now login to yoru vBulletins AdminCP and see if the issue still appears.

Reply back on how it went. Good luck

Try overwriting the file using your cPanel file manager or Plesk file manager. Sometimes I've found that using Filezilla does not always overwrite properly. So if you login to your hosting account that uses one of those or if you have something else that allows you to upload files try uploading forum.php that way. What is happening is your file is not getting overwritten. I've seen this same thing on a client's server. He has the host from hell and weird things like this happen all the time.

Thanks for your replies guys, I've done everything above but nothing work , I don't get why only /admincp/forum.php is the only file that it's not working , everything is working great , all the admin panel is fine , it's only this forum.php which is not ok when trying to add or manage a forum

Try getting your host to overwrite the file. They may have better luck on their end.

bump this issue , anyone can help me , I can give you access to check it yourself @ozzy47 , you're not replying on skype or to my pms. Thanks to everyone who is trying to help

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I've solved the issue myself , thank you