Thanks. Looks like VHE and Jackhammer treat the DOCINFO (info about editor cameras) part as optional but Sledge won't open the file if it's not present. As a temporary fix to get the file to open in Sledge:
1. Open the file in a hex editor, for an online one try:
https://hexed.it/2. At the end of the file, add these bytes:
44 4F 43 49 4E 46 4F 00 CD CC 4C 3E FF FF FF FF 00 00 00 00
To do this in the online editor, open the file and scroll to the very end. There will be a box with a "+" inside it. Click the +, copy the text above and press Ctrl+V to paste. In the dialog that appears, select "Hexadecimal values" from the drop down box and press "Apply", then export the file.