Once I try to log in on Magento Connect this error message appears
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at firstname.lastname@example.org to inform them of the time this error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.
My version is 126.96.36.199
Does anyone know the possible solutions?
There are dozens of possible reasons. As it is said in the error message, "More information about this error may be available in the server error log.". That means you have to look into php log on your server (do not confuse with Magento logs system.log and exception.log). If you do not know where it is located, your hosting provider should be able to help.
Try checking if your file permissions are set correctly, especially for the "/downloader" directory.
For a server with SuPHP / SuExec, all files should be set to 644 and all directories to 755. Otherwise some of the directories will need to be set to 666 and 777 (which can be insecure).
You may need to contact your hosting provider to know the correct file permissions to use.