Here the case: I have a plugin installed in .qgis2/python/plugins that remains broken and I can't make it work. I've found it here https://github.com/volaya/mgrs-tools. If any of you know the way to fix it I'll be so glad.
Hint: from forum I've heard it can come from dash characters in the plugin's folder name.
I use Windows 7 64-bits and QGIS 2.6
It appears that running QGIS 2.10 32-bits solves my problem. Meanwhile as the issue persists with 64-bit versions of QGIS I've just partly found an answer.