Blank page on single post for posts with certain tag

I have a site with a ton of pics of girls in bikinis (idolx.net)
I had my theme altered by a developer and am since experiencing the following problem:

Posts tagged with a certain tag are showing up blank.
This tag has over a 989 posts (idolx.net/tag/%E6%B0%B4%E7%9D%80/)
(over a thousand posts if I include scheduled posts)
The posts show up fine on the tag page, but the single post itself is blank.
Reverting to the old theme before modifications fixes the issue.

I’m guessing somewhere in the altered theme files there is maybe a tag limit of 999?? or something?

If anyone could point me in the right direction or give hints I would appreciate it; the freelancer that did the modifications is not being very responsive.

Thank you