The crash dumps themselves are not too revealing so please provide some information which plugins caused the crash.

Any x64 synthedit plugin. In my case: Kx Polymod, Aegis, Sk7, Synlay, Lynx.

Here the plugins only crash here when trying to open the editor after having opened another SynthEdit plugin's editor. The crash happens deep inside effEditOpen so I doubt there is something we can do about this. However, forcing the plugins to run inside the plugin bridge circumvents the problem.

I contacted the SynthEdit author as the issue even happened when I created a SynthEdit plugin that consisted of nothing but a knob.

I got no reply from Jeff yet, but I was able to verify that not calling FreeLibrary when unloading the plugin prevents the issue from happening. Apparently Reaper even has an option which I guess does exactly that. While it would fix the problem, not calling FreeLibrary would of course be a rather dirty workaround that would potentially prevent memory used by unloaded plugins from being reused.

I confirmed that the same crash happens in Reaper if you enable "Allow complete unload of VST-plug-ins", so it's not an OpenMPT-specific bug. This really is something that must be fixed in SynthEdit. Maybe we can add an option to OpenMPT to never unload plugins, but I don't think this should be enabled by default.

As there are no news from the SynthEdit developer, OpenMPT will now (as of version 1.29.00.42 / r12445) no longer call FreeLibrary anymore to avoid these crashes. For users concerned with resource consumption (mostly RAM) from plugins that are no longer fully unloaded, there is a hidden setting VST Plugins.FullyUnloadPlugin which can be enabled to return to the old behaviour (including crashing SynthEdit plugins, obviously).

The bug was fixed in a recent SynthEdit version (sometime between build 433 and 442), so I changed the default of the newly added VST Plugins.FullyUnloadPlugin setting to 1 in r12519 again. This means that with the default setting, those faulty plugins will crash again.

I recommend that you contact the authors of plugins you care about and advise them to recompile their plugins with the latest SynthEdit version, as this is a more practical and future-proof approach than keeping the setting in its non-default state.