First, try checking your apache error logs not the magento ones.
All three times I’ve encountered this issue it has been caused by the mod_security apache module breaking the script by blocking access due to a false rule match. The mod_security apache module is notorious for causing similar issues with most major CMS when all the rules are updated, having to disable a rule or two for compatibility is pretty common.
If your client is on a shared host you can send them the line from the apache error.log and they can remove the offending the rule for you.
If this is your own server or a vps you can fix the rule yourself, these instructions are a helpful place to start:
I hope this helps. Best of luck! Thanks.