Urgent: cPanel backups failing with 'disk quota exceeded' error despite ample space, breaking our server management workflow!

Hey everyone, really stuck here and hoping someone has seen this before. i'm running our 'Website Maintenance & cPanel Management Services' and dealing with a critical issue that's completely halted our backup process. this is wrecking our server administration efforts.

• The Core Problem: cPanel backups are consistently failing with a "disk quota exceeded" error. it's popping up every single time a backup tries to run, which is just insane.
• The Frustrating Part: My server actually has plenty of free disk space (checked via SSH, cPanel disk usage, WHM). it's showing ~400GB free out of 500GB total. like, what's going on?
• Impact: This is breaking our entire server management workflow and making me extremely nervous about data integrity. we can't operate like this, especially for our clients who rely on our website maintenance & cpanel management services.
• What I've Tried So Far:
  • Checked cPanel disk usage tool – shows plenty of space, like 80% free.
  • Logged into SSH and used df -h and du -sh * – confirms ample free space, no weird large files or directories hiding.
  • Checked inode usage – well within limits, not even close to full.
  • Verified user and reseller quotas – none are set to restrict this much, not globally or for individual accounts.
  • Restarted cPanel services (cpdavd, cpanel, etc.) – no change, error persists.
  • Checked permissions on the backup directory – they seem fine, owned by root, 755.
  • Contacted our hosting provider's basic support, but they just pointed to the disk usage tool, which isn't showing the problem. they weren't much help, honestly.
• My Desperation: i've been at this for hours, trying to figure out why cPanel thinks there's no space when there clearly is. Is there some hidden quota, a corrupt file, or a specific log I should be looking at that isn't obvious? this feels like a phantom issue, and it's driving me nuts.
• Seeking Specific Advice: What specific commands or diagnostic steps can I take to pinpoint what cPanel is misinterpreting? Could it be a filesystem issue despite df -h looking fine? maybe some obscure cpanel config or a kernel-level quota i'm missing?

Anyone faced this before?