MSDB transaction log was full due to 'CHECKPOINT' despite disk has space. Had to rebuild from scratch

Today I came across something new in my 20 years of DBA life - MSDB transaction log was full due to 'CHECKPOINT'

Due to the secure environment I'm not able to copy-and-paste out anything but you'll get the idea

Issue

• We had a customer disk running out of space, at 0 bytes, it caused everything to stop
• After some cleanup and emergency actions
• Removed pagefile
• Emptied Recycle Bin and disabled saving space for the drives
• We managed to free up 20~80GB+ space, yet the SQL Agent still won't start up, and the error log/event view shows - MSDB transaction log was full due to 'CHECKPOINT'

Things Tried (and did NOT work)

• Obviously restart SQL Server and Agent
• Put MSDB into Emergency mode - nope, not allowed
• Rebuild Log
• Backup Log
• Adding new Log file to MSDB
• Change MSDB recovery model (from SIMPLE to FULL or anything)
• Reboot the physical server
• ^ all above made sense because the LOG is full and cannot do any action

Resolution

• Had to rebuild MSDB using this method
• https://sites.google.com/site/sqlservermsdn/rebuild-msdb-database
• We lost the Backup plan, all SQL Jobs
• had to re-create Backup plan using Maintenance Wizard
• created the jobs elsewhere and paste and re-create

Things to Do AFTER or Prepare beforehand

• Script out your SQL Jobs if possible
• Backup if possible, for restore AFTER the rebuild
• AFTER - had to re-grant permissions to MSDB on SQL Logins