If you are not 100% sure that error 413 is crashing conversion process, then submit ticket with admin access details inside message, so our technician could confirm it.
Error 413 should also leave entries in server error log file.
If you already know, then there might be 2 possible explanations & solutions:
1. Request entity is too large.
For Nginx server increase client_max_body_size 100M value should be fine.
Apache server, increase LimitRequestBody 104857600
2. ModSecurity: Request body no files data length is larger than the configured limit.
Increase LimitRequestBody or SecRequestBodyLimit in Apache settings (104857600 value should be enough ).
So far experienced on: Media Temple DV w/ SSDs (CentosOS 7) - Level 1 hosting plan.
Those are server settings and can be change only by server administrator. Contact your hosting support and ask for increasing those values.