Randomly newly created users failed to join VPN with "authentication failed"

Hello,

I have a users's group that is enabled in  "user portal" and one "SSL VPN", meaning all users that belong to that group, will have both access to the user portal and the one SSL VPN.

I have a small script that generates user credentials for new users, which I later copy & paste on the Sophos' web portal,  to create new users (my point on this is that ALL users are created in the same way).

While this usually works... lately some new random users have the following problem:

1) the CAN log in to the "user portal", in order to download the sophos' VPN client.

2) the can NOT log in into the VPN, after downloading the software. The error in UAC is: "Authentication failed"

I tried changing the name (hence it will re-create a certificate, I guess) and changing passwords, but if it's similar to the ones before, it won't work... so I need to keep creating new users, test them (by downloading the new SSL VPN config file and try to log in) until a set of credentials works... 

Again, the users are created the same way and the data (credentials) are created using the same patterns (length/characters of password, user name, etc)

So, any thing I could try in order to avoid that workaround?

Thanks in advance!.