So... what happened was that I added "pokecube:pokecube" to the whitelisted items. I was using Tool Belt version 1.16.3-1.15.2. I expected it to load and let me put the whitelisted item in. Instead, the server crashes while trying to load. This is a link to the gist bound to the report generated: https://gist.github.com/TeamEeveeG/7a70e3617e311cd1079cbbd96a7032a6

Here's the thing, pokecube:pokecube exists as an item, as shown in the screenshot below:

Is it possible you added it without quotes?
The whitelist should look like [ "pokecube:pokecube" ]