I’ve made some research and seem to manage this issue.
I found that is_executable() function is not always reliable for checking directories.
I’ve changed 2 lines of code:
includes\PHP__Snippet_Functions.php
line 76
if (!is_writable($dir)) {
controllers/settings.php
line 31
elseif(!is_writable($snippet_dir)) {
And now it works under my Windows OS.
It has to be checked under Linux/Unix OS.