This issue is caused by one or more converted pledge exceeding 150 installments. CRM has a hard coded limit of 150 installments for pledges.

To resolve this issue: 
1) Go to the selection used in the Global pledge write-off process.
2) Add "Number of installments" to the display fields > Run the query to find the pledges with 150+ installments
3) Edit the pledge(s) to reduce the number of installments to 150 or less.