Managed to have the custom error messages removed from the server and here's the error we're receiving now:
Server object error 'ASP 0177 : 800401f3'
/WA_Universal_Email/CDONTS_NewMail_VB.asp, line 92
Seems to be a problem with the CDONTS_NewMail_VB.asp file.
How can I resolve this issue?