Some very old donation forms may experience this issue. If there are donation levels including Other, the Other level is not processed properly. Deleting the donations levels along with Other and recreating them on the donation form resolves the issue.
  1. Select Fundraising > Donation Management
  2. Select All Donation Forms tab
  3. Find the donation form and click Edit
  4. Go to 2. Configure Donation Levels
  5. Unpublish the existing levels and create replacements for each level