If blowing plug-in cache solves the problem, then your plug-in must have returned an error during a previous launch; I've asked Zac for comment.
Do the customers experiencing problems also have multiple sets of plug-ins installed besides yours? I wonder if the problem you're seeing is the same as this one:
If the customers have multiple plug-in sets installed, you could ask them to try the same workaround:
As a workaround, try moving the set(s) of plug-ins that aren’t loading to outside of the plug-ins folder, then hold down Shift while launching to clear the plug-in cache. The plug-ins still in the plug-in folder should okay. Then move the problem plug-in set(s) back into the plug-ins folder, and launch (without holding Shift this time). This means the plug-ins re-added will be loaded incrementally. The plug-ins should load successfully in this case, and should be fine after that.
Please let us know if this helps.
Hello and thanks for your answers. In most cases it is necessary to make changes in the registry what is not very user friendly. When I look into the log-file it says that the Plugin was not loaded because it is set to ignore...
The workaround of Zac worked in very few cases only...
Thanks for your help,