I'm getting an Error 500 result when trying to produce Thank You letters for contributions since upgrading to 4.7.10 (and Drupal 7.50). Other Print/Merge operations are functioning normally. Backtrace and debug do not produce any additional information. The error comes ~1 second after clicking "Make Thank-You Letters"--seeming as though it's not even processing the information.
I've eliminated the following variables as the cause: Number of records, which records are selected, document type, thank you document body, page format, grouping.
I read that Error 500 is often a permission error. Everything seems correct, but I'm no expert in that realm. Tried the sandbox demo and it's not a bug there.