The following error occurs while trying to login to any webmail client :

Internal Server Error
Unable to load featurelist ‘default’: Permission denied

Fix :

1. Log in to the server via SSH as ‘root’.
2. Open the file /etc/cpupdate.conf and make sure CPANEL=manual.
3. Run /scripts/upcp –force

This should fix the issue!

While trying to send email, following error occurs in Horde/Squirrelmail :
535 Incorrect authentication data

Fix:

This usually occurs when the option “Allow mail account authentication using the password of the domain owner’s account” in WHM -> Server Configuration -> Tweak Settings is enabled.
Webmail logs you in but attempts to use the cPanel account password via SMTP. Hence. the problem.

Disable the option and check.
If this does not fix the issue then try running /scripts/upcp

When an user logs into webmail , he should just have one email client ( say roundcube ) whereas all other users should see all the three.

Solution :

Suppose the username is ‘xyzco’ for the domain xyz.com.

1. SSH into the server as root, and edit the file /var/cpanel/users/xyzco

2. Now, if you want to disable all webmail applications and enable roundcube mail, you would need to add following lines:

skiphorde=1
skipsqmail=1
skiprcmail=0

You can specify the values as per your requirement.

3. Then run the following command :

/scripts/updateuserdomains

4. And restart cPanel :

/etc/init.d/cpanel restart

Now, when the user xyzco logs into webmail, he will only see roundcube mail, and not the other two