The system user has exceeded its allotted quota of disk blocks.

Symptoms

The following error is generated when attempting to log into webmail:

Internal Server Error
500

The system user “USERNAME” has exceeded its allotted quota of disk blocks.

Description 

This is caused by the noted user account exceeding the allotted disk quota.

Workaround

Reduce the user's disk usage or increase the user's available quota using WHM's Quota Modification module.