Recently I was trying to tidy up my home made plugins. I opened one for editing made the changes and then got this message
However, FH was happy to save the plugin with a different name. After a bit of investigation I found that the problem was due to the hidden attribute having been applied to some of the plugins in the Program Data folder. Turning this on and then off established that this was indeed the problem so simply solved. I have no idea why the hidden attribute was applied but I assume it was when I was restoring the plugin folder after a Windows 10 crash."A problem occurred. Task may not have completed correctly".
But this does not answer the question as to why this causes FH a problem.