When tusers report that they cannot login to the DRM portal, first make sure that they are in fact logging into the correct portal. For example DEMO accounts are hosted at artistscope.net/drm/ while real AUTHOR accounts are hosted at copysafe.net/drm/
You can verify that they are in fact logging into your portal going to Stats > Logins. If they are not listed there then they have misinformed you.
However if they are listed you will see if they were successfully logged in or not and when. If login failed, you need to investigate why. Common causes:
1. Their account has expired or been suspended by admin.
2. Their user group has expired or been suspended.
3. Your admin account has expired or been suspended.
4. Your group settings do not allow user logins to the portal.
Hint: as ridiculous as it sounds some inexperienced admins have locked everyone out of the portal by suspending their own group.