The installation of .dll's into the plug-ins-subdirectory by additional plug-ins can cause malfunction of others.
Submitted by N.M.
Bug 756922 discovered a general problem: The installation of a new version of the „libstdc++6.dll“ by the "GIMP_Extensions_v2.8.20150403.exe" (GIMP Extensions Pack for Windows) into the plug-ins-subdirectory had caused a malfunction of the standard plug-ins "file-pdf-load.exe", "help-browser.exe" and "webpage.exe".
These .exes obvisiosly try to access first the .dll in their own subdirectory /lib/.../plug-ins and find there the version of „libstdc++6.dll“, which the Extension Pack had added. So they cannot find their right „libstdc++6.dll“ in /bin
I recommend to unable this kind of conflicts either by protecting this and other sensitive pathes or by precautions (f. ex. prioriting schemes) in the standard packs.
I would spend a special subtree structure to non-standard plugins.