I could see this happening if an account was used to process the Internal Usage transaction but was then deleted.  

I'd do a sanity check to ensure all G/L accounts used in I/C Accounts Sets, Categories exist and are active.  Would also be worthwhile checking the Location Segment Over-Ride to see if something looks askew there.

Other 'hail Mary' items to check:
1. Fiscal period is not locked
2. Optional Field consistency between what's mapped in I/C and what's setup in G/L
3. Posting user has permissions for required accounts (if GL Security by Segment is enabled)

I could see this happening if an account was used to process the Internal Usage transaction but was then deleted.  

I'd do a sanity check to ensure all G/L accounts used in I/C Accounts Sets, Categories exist and are active.  Would also be worthwhile checking the Location Segment Over-Ride to see if something looks askew there.

Other 'hail Mary' items to check:
1. Fiscal period is not locked
2. Optional Field consistency between what's mapped in I/C and what's setup in G/L
3. Posting user has permissions for required accounts (if GL Security by Segment is enabled)

Thanks, it helped.