If the error relates to just loading, then it may just be a matter of placement and command content. Where are you placing all of the required files? In classic, it is:
\AGK2\Tier1\Compiler\Plugins\PluginName
For the 'commands.txt', 'windows.dll' (32-bit), and 'windows64.dll' (64-bit).
For Studio, it is:
\AppGameKit Studio\Plugins\PluginName
For those same files. So I'd suggest verifying all three files for every plugin is in the correctly named folder (foldername must match the plugin name exactly).
If the loading error relates to platform, make sure you have specified the correct Win64 string for every command in 'commands.txt' and that the 64-bit version of the DLL is exactly the same as the 32-bit (no mismatched decorations, AppGameKit pointer in place, etc). Otherwise, you could wind up with a situation where you're loading and running on 32-bit in classic and it works fine, but then trying to do the same in Studio in 64-bit doesn't.