This is a plugin-specific issue, so please name the plugins that aren't loading.

Since you specified "32 bit and 64 bit" as a platform, I can only make guesses and suggest that for the time being, you use the OpenMPT version that has the same "bitness" as the plugins you're using - e.g. if the plugins are 32-bit plugins but you are using a 64-bit OpenMPT version, consider using the 32-bit version instead so that the plugin bridge doesn't have to be used. It's most likely the reason for the plugins hanging.

I tried some of the plugin which would supposedly not load and I cannot reproduce the issue here. Can you please narrow down the problem by removing one plugin after another from your files until the problem doesn't happen anymore? That would help a lot tracing the problem down.

I have made some modifications which should hopefully help tracking down the crashes in the plugin bridge.
First, download the current OpenMPT test build from http://buildbot.openmpt.org/builds/auto/openmpt-win32/openmpt-win32-1.24.00.04-r4287.7z (or http://buildbot.openmpt.org/builds/auto/openmpt-win64/openmpt-win64-1.24.00.04-r4287.7z if you are using the 64-bit version)
Then, copy over these special versions of the plugin bridge: http://sagagames.de/stuff/pluginbridge-test.7z
Whenever the plugin bridge crashes, it should create a memory dump in %temp%\OpenMPT Crash Files. The file names of these memory dumps will be something like "PluginBridge 2014-09-13 12.34.56.dmp" - please put all these dumps in a zip file and attach them to this bug report.

Two days ago I've experienced similar issue (perhaps the same), but at separate instances of OpenMPT. You can crash the plugin bridge if you load a song with Sidekick v3, in each instance.

Yes, SideKick doesn't work when it's being loaded into two different processes. This is a well-known issue. The problems reported here may or may not be caused by similar plugins, which is why I need more debug information.

Closing since no more information was provided.