Upgrading 1.6.16 to 1.8.7 Failing with 500 Error

[Devilshakerz]

The size of that mybb_posts table is about 1GB. So do I just have too many forum posts to be able to update?

It might be too many to be done on your server - the 1.6→1.8 upgrade takes significantly longer than minor upgrades, mainly because users' IP addresses are being converted to a different format. The MyBB installation is most likely exceeding the time/resource limit.

You can try dropping data related to IP addresses in posts (make sure you have a backup):

UPDATE *posts SET `ipaddress`= ''

and trying the upgrade again;

nonetheless you might want to try the upgrade on your computer with disabled time and resource limits (it can take a while) and uploading the converted installation to your live server.