Fatal error: Cannot unset string offsets in... on line 141**** uploading pledge payments

When uploading pledged contributions I get the following message: *Fatal error: Cannot unset string offsets in /home/sbsadmin/public_html/sites/all/modules/civicrm/api/v3/utils.php on line 141*** This is on a system at 4.6.14 - see attached screenshot.
Some contributions work, but it then crashes - it's hard to see any pattern. There's no traceback or any error message left anywhere that I can find. I can reproduce this problem on a test system at 4.6.12. On the 4.6 demo site I tried the same but modified input file and ended up with a white screen.