when people try to register they get a (internal server ) error but they still recieve an email with username and password and all that works just fine. i didn't change anything recently and this problem came all of a sudden . could this be a chmod problem in the ubbmisc.cgi ?

Yes It can be. Or maybe you have stuffed the coding in ubbmisc.cgi when you installed a hack.

Have a look at the servers error.log and it will tell you where the script is causing the error. This way its much easier to diagnose the problem!

seems to be the ubbmisc.cgi , what is the correct chmod for that file ?

All CGI files should be 755. Variables files are 777.

I wouldn't think that ChModding would be the problem..could be..

well to my knowledge i did everything to get that problem out of the way , but it is still there , even when i upload a backup copy of ubbmisc.cgi , i just can't understand why members get this error and still the username a password gets send to them . could it be some zero bytes file problem somewhere ?

Check the size of your members files, if any are 0 bytes then they will get an email with no user/pass in it. I had a similar problem when I ran out of disk space in my server.