I'm not entirely sure if one could consider as a bug but here is a description of the problem.

field name="markersDirectory" type="folder list" ... directory="images"

For some reason user created a directory in administrator/images.
That way the validation of the directory in "libraries/joomla/form/fields/folderlist.php:35" "!is_dir($path)" do not match as the relative path is "administrator/images" and this directory indeed exists.

I'm not sure how the xml form field supposed to work but I think it should always contains a path relative to the Joomla! root path so it would be probably better always to prepend the JPATH_ROOT constant.

Thoughts?

Regards,
Radek