Multisite Conversion White Screen Error

Since you’re not getting anything in the page source, it’s at least reasonable to work under the assumption that something is either going wrong in the initialization process or very early on in building the page templates (like something you’ve hooked into wp_head().

I would try adding “breakpoints” to any functions that you’ve hooked into the init or wp_head actions and see how far you get.

I usually echo something simple at the beginning of each function:

echo "<h1>APPLES</h1>";

I use a different fruit each time so that I know I’m progressing deeper into my code. When you don’t see the output, you’ll know you’ve zeroed in on the cause of your issue.

There are some additional ideas here:
White screen error for a custom theme