[quote user="csouza"]I am able to negotiate the AVXATTR_Effect_ModalUI attribute to open a UI using a ACFCompType_ModalUI component. It creates a button on the effect editor panel with a button in it. [/quote] This is the only safe way to display a model UI from the Effect Editor. If you need more model UIs, then there are many options that

You are experiencing https://en.wikipedia.org/wiki/DLL_Hell. Media Composer has it's own version of the Qt DLLs & framworks with custom patches to Qt that are not necessarily binary compatible with any externally built Qt-based AVX plugins. Your AVX plugin is loaded into the Media Composer process, so it cannot also load DLLs from an externally